My Batman game would center on three experiences:
Combat
With a fighting system more akin to a fighting game than an action one, with a handy lock on mechanism, you have access to a plethora of great moves as well as a plethora of free-form on the fly group oriented combos as you leap into large rooms full of unlucky evildoers and open large cans of whoopage. Of course, thugs come in all shapes and sizes, and you'll never know when someone might pull out a gun, or a crowbar, or some kung fu... all crowbar weilders don't wear the same jacket, that's for sure...
Detective Work
This is where RPG aspects would come in... while each "case" (think levels, but free roam around all of gotham city) would have a beat-em up combat path to win, it would also have a detective work path to open up the 'final conflict.' Batman when pressing the black button the x-box, or perhaps L2 on the PS2, would go into detective mode... from here he would have access to various intimidation "attacks" and an action button used to sample clues, open/investigate/examine items of interest and a talk button with branching (multiple choice) conversations for parlaying with police, victims, commisioner gordon, etc, etc... and of course, a Bat-Journal for keeping track of your clues, evidence and implications... complete with detective-show like flashbackish "visions" of how the crime could have occured as you put things together...
Stealth and Atmosphere
Batman is all about his environment... he's all about stealth, and even though players want that to be a big deal, most DON'T want it to interfere with them treating Batman like a brawler... which is fine and doable... make the highways dark... the obvious and most easy paths through Gotham (for Bats anyway) are dark alleys and rooftops, easily traversed... the bright populated areas of gotham cut off by good game design, though still visitable, especially later... Give players points and "XPee" for fading away without sight after a conversation, or taking out 9 thugs without being seen and then appearing before the tenth one... make it a big deal, without making it a burden... allow the player Splinter-Cell like Stealth ways to complete missions within a case as well, flicking switches, taking advantage of the environment, and otherwise not having to lay a finger on anyone and switching the final fight in your favor... (or at least onto even footing)...
this gives the player THREE ways to play the game... using one will be fun, but using them all at once brings the most success and bonuses for getting all the bonus objectives...
Sub-Games (The details that take it from being a great game to being a classic experience)
Bruce Wayne -- By taking the batpole upwards you access some Bruce Wayne gameplay... parties, dates and business. Though these might seem like minigames (which are not uncommon nowadays)... business serves as a way to get new equipment for your character, parties serve as a way to get top-down information on what's happening all over gotham and dates, well... they're just fun...
Driving -- Top notch. Realistic Physics, trick out your handling, acceleration and top speed, several 'tracks' and 'obstacle courses' are cleverly built into the city and are used for chase sequences at certain parts of the game, regardless, the Vehicle can be used at any time, it's just that driving it on the street attracts police attention... the vehicle has a stealth mode of course... Brucey can also drive the rolls royce around the city...
Prep-Time
No Batman game is complete without it... Prep-time is really a fusion of the options screen with the detective game as 'fuel' but as you develop info on your perp or the people you are going to assail, you'll get suggestions on what to equip against them in the bat-belt and tumbler loading screens. Furthermore, as you press on against each unstoppable clayface (each one possesing less weakness and more power than the one before it, well, all except the dud...) you begin to master the system which Batman uses to take out people like Bane, Superman and his writers... Prop time also includes a preview fight mode with dream-sequence borders as you test out your combat tactic 'in your mind' before you leave the cave with your items...
Storyline -- This will be THE Ultimate Joker story. The game will be titled the same as the movie sequel to Batman Begins... and like most videogames it will adopt the tale of the movie, but this game will add in levels and storyline based on the comics... The Killing Joke, Mad Love, The Man Whole Laughs (that the one with the fish, right?), A Joker Christmas, Dark Victory, The Long Halloween, ALL of these will be taken into account in this story. The Joker will play a huge part. Harvey Dent and later Two Face will play a huge part. Falcone. Even little Grayson and Barb Gordon will make meaningful appearances, not to mention tension with Lucious Fox, the death of Gordon's wife, Alfred's great lines and some believable closure with Rachel. As added features, the stories of all SIX Clayfaces will make appearance as you follow the growth of this ugly disease-like condition through the livse of six different individuals... just when you thought the case was closed... it'll open again. And remember, each case is not only a slugfest of locations to discover, but a network of people to investigate and a world of darkness to explore.
Also, when you take out the Joker "once and for all," giving closure to the storyline and game, you still get to keep playing, dealing with re-emersions of Two-Face, Clayfaces, Charaxes, Scarecrow
Training -- This is one of the things that makes the game magic... on top of the sixty hour storyline sprawling above, there are twelve, no less than twelve mini storylines to be played out, each coorresponding to a different type of gameplay Batman does. Six storylines are part of the extensive training mode. Six are secrets that you unlock as you play the game.
Combat-- A training storyline with World Champion Pugilist Ted Grant, in which young Bruce Wayne has a fight and a fling with fellow tutee Dinah Lance. Another, openable, hidden storyline has Bruce Wayne teaming up with Green Arrow in a special multiplayer free-roam adventure against a would-be-gangster called Black Mask and his army of goons.
Detective -- The training storyline has a young bruce wayne in Chicago under the tutelage of not-yet-replaced-by-a-martian detective John Jones in a case worthy of a classic novel that ends in John's death. The other "secret" detective story introduces the "Oracle," a young Barbara Gordon, solving cases with a lot of electronic networking and coorespondence with a certain broody bat as they discover what's up with Charaxes.
Stealth/Travel -- The training storyline has a young Bruce Wayne taught the beauty of the circus, and the art of free running by young couple John and Mary Grayson, while another storyline has young Rich Grayson donning a red vest over his black gear in order to take out Zucco "once and for all"
Bruce Wayne -- The first storyline has a childhood bruce wayne navigating old gotham alongside buddies Tommy Elliot and Harvey Dent while another has Bruce Wayne matching social wits with the smitten big name actress Silver St. Cloud and business sense with the dispicable mogul Lex Luthor.
Driving -- The training session involves a go with the wonderful Roxanne "Rocket" Rodriquez while the "secret" storyline involves you solving a case that makes use of a high-tech boat... yes, it's painted black...
Prep Time -- The training session is a flashback to Ra's temple atop the mountain as you learn to face augmented animals of death, superior combatants like Sandra "Lady Shiva" Woosan and eventually your predecessor (as leader of "these men"), the man who becomes Ra's after Watanabe dies... The secret unlockable quest is when Bane comes to town, and not only is he practically invulnerable on this scale, but he can out prep-time you if you're not careful.
Other Modes
While the main game and training can be fun, adding a few more modes of gameplay can REALLY crank up the replay value...
Versus -- I would be remiss otherwise... but the lock on feature gives you two attacks, a special and a block button. That's a fighting game, ladies and gents and you can squab it out as Batman, Joker, Two Face, Clayfaces, Ted "Wildcat" Grant, Dinah "Black Canary" Lance, Oliver "Green Arrow" Queen, Ra's Al Ghul and Lady Shiva.
Racing -- Again, how could we pass up testing the Tumbler against Bruce's Rolls, or his Ferrari? What about Joker's tricked out trumped up mustang? A gotham special unit Squad Car? And as secrets, Barbara's dirt bike on nitro and Harvey Dent's Beemer...
Time Attack --Take out the most bad guys in x amount of time? With infinite batarangs? Without being seen? Various mode exist here...
Multiplayer
I love it too much to leave it out... secret characters abound... split screen, network your boxes, whatever, it's all for the love of Batman... but none of the characters can outshine Batman, but on the contrary, it takes three others to make up what Bats does, and in a four-player co-op game, you have three classed chars and one multiclass character... which still works in a way...
Barbara -- Can be a detective just as good as Batman, though with weaker 'intimidation' levels... she gathers clues and info, just like batty... very limited fighting...
Grayson -- Can be a stealth and acrobat just as good as Batman, though he lacks the ability to 'fly' he does glide well... very limited fighting...
Green Arrow -- Once opened can be used again and again, at any time... very good at fighting and possesses a large amount of varied 'items' as arrows/ranged attacks, is not much of a detective and not much of a stealther
(I just realized how long this is)
Extras and Rewards
While cutscenes used to cut it... that's CLEARLY not enough anymore... people want some nostalgia, right?
Cars -- An old man sells antique and concept cars, many of which are actually old TV, comic and movie versions of Batmobiles in the years gone by... all can be purchased and driven for a sufficient amount of money/mission completions/whatever.
Costumes -- A costumer sells holloween costumes for this 'batman' character, all of which resemble the comics, movie and TV costumes of days gone by...
Suveniers (sp?)-- Not only can the above be displayed in the cave, but each case brings 1-3 special items to be displayed in the cave, depending on how it was completed... the museum area of the cave can be viewed in first person mode as you walk around the cave, and each solved case has an equipable item for your belt or car (combat solution), a large cavescape changing item (detective), and more free-running moves (stealth solution) from unique areas in the game... of course, each case can be solved all three ways at a time... and no matter how you defeat it, each case brings new comic covers and stuff onto the 'batcomputer'
Um... I think that's about it...
Yeah... I really can't think of any other way to make a Batman game better... all of you guys have great ideas tho...
Oh... I know!
The Ultimate Unlockable
If you are just fool enough to get 100% on this game... which should take you well over 100 hours between all the missions, side missions, versus games, time attack completions and special encounters (Dr. Leslie Thompkins, Duela Dent, Lex Luthor, you get to play as the Joker. Back from his 'demise' at the hands of Batman, the man with a multiple choice origin steps onto the scene under player control.
Free roam, with loads of crazy items and toys at his disposal, his own multiple choice of hideouts, a 'prep time' screen that allows him to send out and organize his thugs... instead of interrogating crooks, he recruits them... instead of a bruce wayne life he has an (optional) Harley Quinn "pet" character and once he hits the streets, he has the chance to slay everything that moves as they scream... and a laugh button, which does the same thing for Joker what striking from the shadows does for Bats... instant fear.
From here, players can play multiplayer free roam competitive games... or go solo with the Joker. This also opens up all-combat Clayface fusion, minion focused Two Face and sneaky little vixen Harley Quinn for four on four free-roam chaos matches!!! Or just take on the police and, as you follow an all new Joker storyline as he discovers his past... or various versions of it... and not even Batman can stand in the way of his crazed path of death... Of course you slowly unlock peices of FMV mixed up as you hit different checkpoint-kills, but escaping Batman, even the computer controlled one is very difficult, and it gets harder and harder to escape his clutches, and if you are caught, you lose all your progress, and it puts you at the beginning of the Joker storyline...
If you manage to make it through this 3-6 hours of gameplay without being beaten by Batman, then you find out your origin (soon to be revealed in the comics) and kill Batman... only to find yourself in the asylum, the storyline being entirely in your mind... or not... cuz Batman has a cut you remember giving him... doesn't matter... HAHAHAHAHAHAHAHAHAHA!!!!