As far as I think killing Han would have been a great idea, I can maybe understand why Lucas did not want it.

Remember Alien3? Killing Newt and Hicks after all the suffering endured by Ripley to save their skins in the second movie was not a clever move (at least not for me, and a lots of fans).

I think killing Han after all the risks taken by his friends to save him at the begining of the movie would end the same way: the audience would feel like "what? So half the movie was pointless?". The only way for Han's death to be interesting would have been killing him on the Falcon, right after a great heroic move, giving a new hope to the rebels during the final battle.

But as we know, Han is on Endor, and killing him in the destruction of the bunker would not be visualy amazing and we don't have any footage of other characters reacting to that; although seeing Lando laughing right after that (cause he don't know his friend has been killed) could change the mood of that scene... but we're not doing a Grindhouse ROTJ here! ;).

Killing Lando could work indeed, but franckly, as much as I like him, I think he is more a friend to the other characters than to the audience. I would probably be more shocked by seeing the Falcon destroyed... Poor Lando, I always felt he did not have enough screentime (or interesting lines) to really be part of the story (exept his great betrayal/friend move in ESB, of course).

Killing Leia would be a amazing twist, but I don't know how to do that with the footage we got. And I'm not sure we could bring some sens to this.

So, in our quest to find "who must die", we're left with: killing Luke, indeed.

I don't know if it makes sens, but I sure can already see Vader's funeral pyre with the ghost of Luke appearing in front of it. Then he turns and there are the three other ghosts, and everybody smile. A little cheesy, but good cheese.

EDIT: And that could be called "End of the Jedi".