I'll think of The Ultimates as I write this -
Nick Fury - Laurence Fishburne 😖hifty:
Captain America - Matthew McConaughey
Thor - Gerald Butler
Iron Man - Tom Cruise
Hawkeye - Orlando Bloom
Giant Man - Paul Walker
Wasp - Natalie Mendoza
Bruce Banner - Elijah Wood
Black Widow - Kate Beckinsale
Scarlet Witch - Kim Director
Quicksilver - Ryan Reynolds
Betty Ross - Keira Knightly
Well for the first movie I think the cast should be rather small like only 5 or 6 Avengers. Such as:
Capt America- Matthew McConaughey
Thor- Vladimir Kulich from the 13th Warrior he played Bulliwyf
Ironman- Robert Downey Jr. for the sake of continuity. Other wise Karl Urban
Hawkeye- Paul Walker
Giant Man (Hank Pym)- Ryan Gosling
Wasp- Kate Beckinsale
Maybe add in some cameo's by different Superheroes like X-Men did.