I haven't had time to write a story, but the basic idea is the general plot line of the first Ultimate X-Men series.
The X-Men
Liam Neeson as Professor X: He always plays a mentor character, has a great voice, and is the only actor who could replace Patrick Stewart and not be lame.
Matt Long as Scott Summers/ Cyclops: He is a young star on the rise and has the look and charisma to be a good Cyclops.
Gemma Arterton as Jean Grey: She has been sprouting up in a lot of new movies and has that cool voice that would make her perfect for Jean Grey.
Anton Yelchin as Bobby Drake/ Iceman: He has definitely proved his acting ability and would be great for Iceman and Jimmy Olsen.
Zoe Saldana as Ororo Monroe/ Storm: Since I'm going with the ultimate version of the character, do I really need an explanation?
Alan Ritchson as Piotr Rasputin/ Colossus: He definitely has the build and Colossus isn't in terrible need of a great actor anyways.
Nina Dobrev as Rogue: She is a young actor who isn't all that spectacular but she could do well if given the shot.
Teddy Sears as Warren Worthington III: I took this from someoneelse's fancast, but I still think he would be good
Dougray Scott as Hank McCoy/ Beast: He was supposed to play Wolverine originally, so should be a good choice for Beast as well.
Steve Bacic as Logan/ James Howlett/ Wolverine: I couldn't really think of a good replacement, so if anyone has a better idea, I'm all-ears.
The Brotherhood of Mutants
Jeremy Irons as Erik Lensherr/ Magneto: Great actor, great villain role, great villainous role. 'Nuff said.
Kevin McKidd as Victor Creed/ Sabertooth: He has the build and acting capability to be a intriguing henchman that could have interesting conversations with Magneto than being an over-the-top feral brute that communicates in grunts and growls.
Sienna Miller as Raven Darkholme: She has the body and was an alright eyecandy henchwoman in G.I. Joe. Baroness and Mystique are essentially eyecandy and since she did well as one, she can do well as this one.
Roland Kickinger as Cain Marko/ Juggernaut: Since he was the new Arnold Scwarzenegger, he should have the chance to come back as a musclebound again.
Jon Heder as Mortimer Toynbee/ Toad: He is hilarious and could provide great comic relief.
Justin Hartley as Pietro Maximoff/ Quicksilver: He is awesome as the young Oliver Queen, so he shouls be able to match his quick wit.
Camilla Belle as Wanda Maximoff/ Scarlet Witch: She has a cool mystique about her and could bring a sense of innocence to the character.
Humans
Michael Massee as Bolivar Trask: He was an awesome mad scientist on Flashforward and could bring the malevolent genius to the creator of the sentinel program.
Well, that's it! Let me know what you think in the comments section! :)