The Star Wars movies feature such an iconic cast that it's hard to imagine seeing anybody else playing Han Solo or voicing Darth Vader, or in the case of the prequels, playing Mace Windu.
It's a sure bet Samuel L. Jackson's face comes to mind when you hear the name Mace Windu, but imagine a galaxy far, far away where Tupac Shakur played Mace.
It's a parallel casting choice, and probably wouldn't have improved the prequels, but at least he would've had his own hologram before he died!
As for the original trilogy, Han is Harrison, Harrison is Han, but how funny would it have been to see Kurt Russell blasting Greedo and giving Chewie a hard time?
It definitely would have changed the dynamic of the film, but not as much as if Darth Vader had been voiced by Orson Welles!
Welles was Lucas' first choice, but he went with James Earl Jones because he felt Orson's voice would have been too recognizable. Sometimes it's good to be picked second!
Read Actors You'll Never Believe Were Almost In Star Wars here (contains NSFW language)