Yeah, I don't think Lando should die in the final battle, I think Han should, Lando should die during the Sarlacc sequence. My idea (that just came to me) is this: They plant the explosives in the shield generator but they don't go off. If they don't blow up the shield, the fleet would be destroyed and everyone would die, so Han sacrifices himself and detonates the bomb manually. This could coincide with Luke hiding in the shadows refusing to fight Vader, and Luke could feel Han dying and it could be the trigger that makes him go after Vader, thus eliminating the need for Vader to talk about Luke's sister and thus enabling the removal of the whole stupid sister subplot.

But I understand it's probably pretty late now and it would also be hard to pull off (though not impossible).