Anakin Vs Vader?
Although highly unlikely because it would have alot of continuity flaws that would have to be tweeked but I just like the Idea. Mainly being, we have the infamous Volcano/lava pit fight scene, and it would involve Ben and Anakin(who eneter the battle at odds with each other(basically because Anakin has pretty much turned to the darkside but Ben is still trying to save him)) fighting together against someone(I've always liked the name Darth Aliksir(Arabic for solution or cure)) who's wearing a Vader like suit! With Ben getting seperated from the fight kinda like in TPM allowing Darth Aliksir and Anakin to square off. Ben recovers but once again doesn't reach his friend in time as Aliksir gains the upper hand and Anakin is cast over the edge! Then Ben finishes the fight defeating Darth Aliksir, and thinking Anakin is lost leaves them both for dead!
Sidious(or someone working for him) then recovers both bodies! Aliksir is found dead, but Anakin is still alive but not for long and with no time to spare the suit is transfrerred from one to the other! The only thing is if this were to happen on screen, I'd want it implied like; both of their pulses are checked and then all you see is Aliksir's Helmet removed and then go on to the next scene! This would allow for everything that comes later, including the big reveleation in 'Empire'!
Actually the name of Vader could be kept also, so there is no confusion as to the name Vader being used in the OT. So in the OT we see the suit, here the name, and think it's the guy who beat Anakin in the Lava scene. But what actually transpired was that Anakin was given the suit to save his life. He could easily just keep the name Vader for decpeptive reasons, so the rebels and Jedi would think Anakin was still truely dead!
The only big problem with all of this is what to do with Count Dooku/Lord Tyrannus! You could kill him off early in the fim, but it would be improbbale that Sidious had someone waiting in the wings or would have enough time to train a greenhorn apprentice to match Anakin! So one you could make Vader a bounty hunter that Sidious hires and confronts Ben and Anakin. Or Dooku could be seriously injured early in the movie, and needs the suit(Dooku's tall enough). Then with a new suit comes a new name in Vader, along with the possibility of a revelation that Dooku/Vader did have something to do with Shmi's Immaculate birth! He could be a sort of "Force Father" to Anakin, allowing the name Darth Vader to be meaningful to Dooku! Remember he has yet to be reffered to as DARTH Tyrannus, just Lord Tyrannus so a new name once he gets the suit wouldn't be a stretch. So he could be the one in the Vader-like suit in the lava scene I mentioned above, just substitute Vader for Aliksir if you go back and reread it!!
Like I said highly unlikely, but I think I worked out most of the kinks just now, but ya have to admit it would be as cool as all hell to see!!!!