Hmmm...
So, a WW movie:
Jamie Alexander as Princess Diana, Wonder Woman
There are better actresses, I'm sure, and even more attractive ones, but as far as having a solid choice, I think Jamie brings the most to the table, and would be most enthusiastic to do it. She is quite attractive, she does action, and she's a pretty good actress, with some respectable wide-ranging experience.
Jake Gyllenhaal as Steve Trevor, Air Force Intelligence
You want to put Diana across from a male lead who is not only uber-talented, but makes the ladies swoon, and is of appropriate age. That pretty much brings us to Gyllenhaal. He's an okay lead by himself, but he needs a love interest to play off of to shine, really, and he'll always have that here, as he'll double as the buddy/sidekick as well as the love interest.
Leonardo DiCaprio as Ares, the God of War
Now the natural inclination would be to go with a character actor who does a lot of bad guys, and that's usually the easiest way to go. But, every once in a while you can get someone who is a viable leading man themselves, like Liam Nesson, Heath Ledger, Tom Hardy, to play your baddie and it takes things to a brand new level. DiCaprio is obviously talented, and can definitely play dark, and with Django Unchained, he's started doing villain roles now. Playing the ultimate Warlord would be nice for all involved.
Lucy Lawless as Hippolyta, Queen of the Amazons
I understand everyone wants everyone here, from 'supposed to be ageless' 30 year olds to 60 year olds that have played Diana before, but honestly... Lucy Lawless is an incredible actress, who is hot, who played a character that might as well have been Diana before, she's still a popular actress and she's used to working in the ancient-y era. No disrespect to anyone else, but I have a great deal of trouble imagining anyone else doing a better job that Lawless.
Other Assorted Castings:
Young/Teenage Diana: Ariel Winter (Modern Family)
Chancellor Phillipus: Gina Torres (Firefly, Suits)
General Artemis: Sarah Michelle Gellar (Buffy, Ringer)
Etta Candy: Nikki Blonsky (Hairspray, Huge)
Julia Kapetalis: Lynda Carter (70's Wonder Woman)
General Phil Darnell: James Cromwell
Detective Ed Indelicato: Nathan Fillion
Aphrodite, Goddess of Love: Christina Hendricks (Mad Men, Firefly)
Cate Blanchett as Baronness Paula Von Gunther
If a sequel for WW ever comes around you want a villain who captivates, who elevates the character beyond what the writer or director could hope for. Cate is that person, every day of the week, every set she steps on. She's young enough to give Diana a run for her money on pure attractiveness but old enough to seem superior intellectually and experientially. Blanchett herself takes a wide variety of roles, so there's little doubt that she'd get in on a quality superhero script. She is of such importance that the other villains in the film are basically her pawns/supporting cast.
Other Wondy2 Castings:
Trevor "Tre" Barnes: Laz Alonso (Breakout Kings, Jumping the Broom, Avatar(Tsu'tey))
Barbara Minerva, Museum Curator: Milla Jovovich (Resident Evil Series, Fifth Element)
Diana Troy, Dark Angel/Wondergirl: Ariel Winter
Captain Nazi: someone almost purely physical, say, Jason David Frank (UFC fighter, Power Rangers)
US President Alvarez: Edward James Olmos
Hades, Lord of the Underworld: Benedict Cumberbauch
Charlize Theron as Circe
At the risk of her playing one too many wicked witches, Theron is really good, and she's definitely up for superhero movies, and she definitely would bring this home in a big way.
Milla Jovovich as Cheetah
Cheetah has some menace in her, and some cunning intelligence, but mainly, this is a very physical part for an actress with a very intense physicality. That's Milla, and always has been. Perhaps someone else will get in the female action game soon, but for now, Milla has it down to a science.