For the sake of time, effort, and space -- I've decided to not cast everyone in the group. Some like Drax, would need to be in the movie with Thanos now appearing in the MCU. Others, like Rocket Raccoon and Groot are just too insane to not include. All in all, I tried my best to think of who will be in the movie and tried my best to cast them. Don't bitch about me not casting the slew of other members I failed to mention, because I just honestly don't think they'd adapt them. These are the members that make the most sense, in my opinion.
Timothy Olyphant for Star-Lord
Anyone who's watched the amazing show Justified, knows that Olyphant is a very witty and spews one-liners almost effortlessly. I think he could make for a great Star-Lord.
Emilia Clarke for Quasar (Phyla-Vell)
Emilia Clarke is amazing in Game of Thrones. As an attractive, and yet powerful leader -- she has the acting chops and looks for Quasar
Hugh Dancy for Adam Warlock/Magus
Dancy is a great, if underrated, actor. I think he can balance the good of Warlock while holding the rage that would make for a great Magus if Marvel decided to go that route.
Olivia Wilde for Gamora
Olivia is sexy, and looks like she could kill a man if she wanted to. Perfect Gamora casting.
Jason Momoa for Drax the Destroyer
Another Game of Thrones inspired casting choice. Momoa could play the role similar to how Ruffalo played the Hulk.
John DiMaggio for Rocket Raccoon
For Futurama and Gears of War fans, I hope this choice makes sense. If DiMaggio could bring even half of his humour he brings to Bender to Rocket Raccoon then we could have a bonafide hit.
Peter Cullen for Groot
Optimus Prime himself. Sure, his lines would be rather...boring; but I think for a voiceover actor having someone like Cullen would be a definite geek get for the film.