Killing off a mentor/father figure has been done one too many times. That, and I'm still pissed Abrams killed a well liked character in STID. (It wasn't even a good death like Kirk's dad in the first film.) If Luke buys the moisture farm, he comes back a ghost anyway.

The massive fan freakout to Chewie's death in a novel 14 years ago is a good indication of what may happen if somebody bites it in the sequel. They probably don't want to go through that again, so the Wook is safe for now.

Droid death can be cheated, since they can probably put the memory in a new body. Artoo or Threepio getting vaporized or melted beyond recognition is the only permanent rub out for them, but you're back to the Chewie backlash scenario.

Leia dying depends on where she's at 30 years on. Presumably she's in a prominent leader role, and possibly become a Jedi.

Han is the logical one to kill, if he goes out doing what he does best, and it's a noble sacrifice. (No falling off a damn bridge!) Harrison has wanted it since Jedi, and might have even asked for it as a condition for returning.

It's a Kobayashi Maru no matter who you kill. And they sure as heck won't kill Lando. ;)