Do you know how SAD it would be if Marvel came out with a JLA movie before WB did? Pit-EE-ful! Anyway, I'm more familiar with the Supreme Power version, so I'll cast to that.
Hyperion - You want someone with a strong look, a very prototypical leading man, but also someone who can bristle with that darkness and anger and conflict without having to say everything. He goes from looking lost in cornfields to being a force of nature.
My dream casting: Jake Gyllenhall - the version of him that's a great actor (Donnie Darko, Jarhead, Brokeback) not what he usually brings to action movies (Princeof Persia, Source Code)
Hilariously Ironic Casting: Brandon Routh
Nighthawk - Someone who is just scary and oozes power and presence. Someone who can get that 'it's Batman' explanation without having had a movie to establish himself. You want to get one of the superstars here, and let them hit it hard in every scene.
My Dream Casting: Denzel Washington - a bit old, but he can dye his grey and still bring this home in spades
Hilariously Ironic Casting: Wesley Snipes
Power Princess - Okay, lets face it. She's there to be sexy, primarily, and say deep twisted disturbing things. Cast to that.
My Ideal Casting: Olivia Wilde - This is kind of her kind of part, a bit spacey, and I'm sure many would love to see her naked
Hilariously Ironic Casting: Megan Gale... or Adrienne Palicki
Doctor Spectrum - A military guy and generally a surprisingly down to earth and typical action hero, with a lot of stunt work to be had. The colder and cooler the better, and while there are plenty of blonde soldier-types I think a few are stand out.
My Ideal Casting: Daniel Craig - I mean, he plays James Bond, and everything he plays he brings that deep seriousness to the role, which I would love to see on Spectrum.
Hilariously Ironic Casting: Ryan Reynolds
Whizzer - The speedster of the group is young and also surprisingly down to earth compared to the forces of nature in SS's 'trinity.' He's more of an everyguy who gets caught up in all this. There's plenty of young black actors out there to fill the shoes as well.
My Ideal Casting: Anthony Mackie - Mackie's a little old for this, but he's been turning in great performance after great performance. It's time someone rewarded that, and he would knock this out of the park.
Hilariously Ironic Casting: None to be had
Amphibian - Weird mute omnisexual water creature? No sweat!
My ideal Casting: Rooney Mara - I pretty much think she can do anything right about now...
Hilariously Ironic Casting: Adrian Grenier
Skrullian Skymaster - Y'know, didn't have one of these in Supreme Power IIRC. My best guess?
My Ideal Casting: Vin Disel - really just cuz of his voice
Hilariously Iroinc Casting: Carl Lumbly