A messy idea for Boba Fett that wouldn't require as much (any?) new footage, though it's somewhat still disjointed:
Show Fett in Jabba's palace, possibly even on the sail barge (maybe up to the shot of him looking suspiciously at R2 just before R2 goes to the top level to launch Luke's saber). At some point show Slave I in the distance taking off (the shot doesn't need to follow the ship from the surface, it could just be in the distance, then show it in orbit). You could do a similar thing to the mockup of Fett talking to Vader in his cockpit, but instead have a holo of a Bothan and use some alien dialogue from somewhere else in the films (so as not to have to re-record anything), and subtitle it to make it clear who Fett has called. Or just have it go to some Rebel leadership. In any case, Fett doesn't even have to say anything. The scene can cut off before he does. (I like the Bothan idea because it presents a mystery for the audience that is only revealed in pieces, later on.)
Later, have Fett on Endor (a la the old Micro Machines set, haha). Not sure what he should be doing - hanging around on the surface, with Vader, like on Bespin (though we need to generate an actual motive for him to be there). Maybe Slave I is parked on the landing platform. We see him in enough shots to establish that he's working for the empire. Maybe he is the one to bring in Luke (though it would be simpler to have him only be with the officer who brings Luke in - that wouldn't require new lines). When the battle breaks out, we could re-use footage of him from the sail barge battle (that he most likely didn't participate in, in this version), and/or pull in some shots of Jango if necessary.
Then, Fett would be the one to corner Han and Leia at the bunker. I could use the idea of having Han just say "I love you" as his last words, to cover that Leia doesn't have a blaster at this point (though I like the cuteness of her line recapitulating his from Empire). We could have Chewie's AT-ST pull up while Fett and the troopers are saying "Get up," and then the "I love you" line comes (as discussed above). Maybe a shot of Fett here (the footage from ANH and of him pulling his gun in the palace may come in handy here). Then, the AT-ST shoots him and the troopers down. Reveal Chewie as normal.