The Riddler is a criminal. He shouldn't be some FBI agent etc. We've seen the "Who is Batman" version already, so we don't need to see it again.
Exactly why? We certainly don't
need it but if it improves upon previous versions, we should see it. It is a good angle that deals directly with one of Batman's central issues, his concealed identity. It is not false to say that if the secret of Batman's identity goes away, it essentially destroys him.
I say, don't give The Riddler a background, leave it a mystery. All his files on the computer should be replaced with question marks.
Many people have brought to attention the risk in making the Riddler too similar to the Joker. You would elicit comparison with Ledger's portrayal, bump into repetition, etc. Because the Joker did indeed operate in a very similar way to the Riddler in TDK (and, to an extent, also in the comics).
So how do you avoid such downfalls? You try to make them as different as possible. And you try to stay true to what makes them different in their source material. Does the Joker have a past, an indentity? No, he doesn't. But the Riddler does. He is Edward Nygma. And leaving him shrouded into mystery would only, well... be too much like the Joker.
Now, it would be silly to conceal the Riddler's identity because everybody knows he's Edward Nygma, or Edward Nashton, so it wouldn't be a thrill to find out. Yet, to make him someone else would feel like a cheat. You can change part of his background story because his background has never stayed the exact same. But his name and personality and the basics of his M.O.... you can't change that.
And you can't pull that trick they did with Ra's in Begins because well, it's been played out already.
The Riddler is about mysteries but mysteries have two sides. He can be the one creating them or he can be the one trying to solve them (at least the big ones... he can create puzzles, but not the big questions like "who is the Batman").
I vote for him to be the one trying to solve them, albeit creating a few to taunt/distract/oppose Batman. My favorites are death traps. The Joker started them so it's only natural that could kick in and become a thing for copycats and new psychopats to mimic. Or at least that's what one would expect. Then this fancy FBI agent called Nashton decides to exploit that and create a false "new type of criminal" persona, a textbook theatrical psychopat: The Riddler. And "The Riddler" (who doesn't actually exist) creates death-traps with puzzles to lure Batman out. But he's not in total control of it, something goes wrong and people die. And in his obssesion and megalomania, he blames Batman for not being smart enough. He ends up behaving exacly like the persona he created... he ends up lost inside his own monster... mirroring Bruce's own risk of permanently becoming Batman and losing his Bruce Wayne side.
If you ask me, it fits. It fits, fits, fits. In that form, he does every single thing a villain should do: 1. Interact with and present threats to other characters (Gordon, for instance, would have this guy breathing on his neck)
Just introduce him starting with disturbing riddles that the GPD decides not to take serioulsy. Then have an officer fall into a trap somewhere in a dark place. Have The Riddler speaking to the officer from a shadowed area. As the officer begins to die, The Riddler slowly walks up to view his work, giving us a good look at him. Then he delivers the body in a box with a questionmark on it and a twisted puzzle inside for them to solve to get their next riddle. Gordon would secretly get the now wanted Batman to help him solve it and Batman would go to where it leads him. The Riddler would be expecting someone from the GPD but then gets the idea to put Batman through a series of challenges so that Batman would be caught while he steals loads of money and valuables.
One or two officers dying? Riddles that suddenly make nervous an entire Police Force that have seen worse? His ultimate goal is to distract Batman while he... steals money. These stakes are too low and we shouldn't be lowering the stakes, by any standards. That would be seen as boring and lazy.
I have to admit the New Detective idea is pretty good, just not for The Riddler because thats not who he is.
But the problem is that making the Riddler completely faithful doesn't make him ideal as a villain for a sequel that should raise the stakes. In fact, far from that. He's very low-key and inconsequential compared to what we're facing now: the possible final installment of the Nolanverse. Doing things "by the book" is anticlimatic and lazy. The Riddler needs change just as much every other character in any other adaption... it's just that in some cases characters require bigger change. He's one of those characters.
But I find those adaptation ideas so good that he could be, in fact, a fantastic main villain. And we all love the Riddler, but how many great stories have had him as the main antagonist? Almost none. Well, with the proper change he could be now. He would retain his name, personality, M.O., motives, maybe even done his costume at a certain point... and still be the main antagonist. How cool is that?