In my case, this is purely from a casting perspective. Omitted roles would be non "starring" actors. I tried to match the number of actors who got top billing with their equivalent characters, which is why Nute Gunray's actor gets top billing, since Peter Cushing, who played Wilhuf Tarkin, was a "star" of Episode IV. Only added case is counting James Earl Jones, who often went uncredited.
1977: The Phantom Menace
Anakin Skywalker: Shawn Bishop (he played one of Roy Neary's sons in "Close Encounters of the Third Kind" which came out the same year. This was the hardest role to find, as earlier child actors are always elusive)
Padme Amidala: Carrie Fischer (like mother like daughter, basically)
Obi-Wan Kenobi: Liam Neeson (fun fact, he was 25 at the time, perfect age for playing the role. Lucas does like to cast near-unknowns, after all)
Qui-Gon Jinn: Charles Bronson (an older Bronson would have the mystique that the older Liam Neeson brought to the role, grizzled, rough-edged Jedi Master, still wizened enough)
Nute Gunray: Steven Whitmire (voice of Kermit the Frog, for an amphibious puppet villain character)
Darth Maul: Bob Anderson (he was the stunt double for the sword-play stunts, Maul's signature trick. Maul didn't have much personality, so whether Anderson has acting chops is largely irrelevant)
1980: Attack of the Clones
Anakin Skywalker: Mark Hamill (again, like father like son, but Hamill also showed that he has an enormous range as a character actor. He could likely have pulled off Anakin's angst better than Christenson actually did)
Padme Amidala: Carrie Fischer
Obi-Wan Kenobi: Liam Neeson
Count Dooku: Vincent Price (it's like he was made for the role)
Yoda: Frank Oz
Lama Su (Kaminoan Prime Minister): Jim Henson (himself a solid voice actor for the puppetry that the aliens would be in this case)
1983: Revenge of the Sith
Anakin Skywalker: Mark Hamill
Padme Amidala: Carrie Fischer
Obi-Wan Kenobi: Liam Neeson
Count Dooku: Vincent Price
Yoda: Frank Oz
Mace Windu: Bernie Casey
Bail Organa: Tom Selleck.
1999: A New Hope
Luke Skywalker: Jake Gylenhaal (funny story, he's 19 in 1999, exactly Luke's age. Clean-shaven and with his hair lightened a little, he's natural Luke)
Leia Organa: Natalie Portman (mother-daughter thing again)
Han Solo: Nathan Fillion
Obi-Wan Kenobi: Liam Neeson
Yoda: Frank Oz (assuming the original trilogy is now made in "fanboy times" i would assume Yoda would get a token role)
Bail Organa: Tom Selleck (ditto the above)
2002: Empire Strikes Back
Luke Skywalker: Jake Gylenhaal
Han Solo: Nathan Fillion
Leia Organa: Natalie Portman
Carlist Rieekan: Sean Penn (more a-list actors for minor roles, similar to Samuel L with Mace Windu)
Darth Vader: James Earl Jones
Yoda: Frank Oz
2005: Return of the Jedi
Luke Skywalker: Jake Gylenhaal
Leia Organa: Natalie Portman
Han Solo: Nathan Fillion
Yoda: Frank Oz
Lando Calrissian: Jamie Foxx (it's hard to find a black actor who matches Billy Dee's brand of cool. Foxx hits close at least)
Admiral Firmus Piett: Sean Bean (more on the "extra a-listers" side)
Darth Vader: James Earl Jones
Yes, i need something to do with my time... Though we can discuss other aspects of the "what if" as well. Would the movies have become as popular if Episode I came first? Not the episode I we got, mind, but an episode I that told the same basic story: a corrupt galaxy where plutocrats can bully small, democratic worlds, jedi trying to resolve the conflict who encounter a prodigious boy on a backwater world, that sort of thing.
Monster Hunter: pissing me off since 2010.
