... I like my "steal everyone else's ideas and incorporate them into my own" version.

. Which involves Buu appearing in a flashback that (Mirai) Trunks recounts to Yajirobe at the beginning of the final installment, as a means of explaining both Freeza and Namek, the former having kinda ran most of the heroes off of the Earth in the second movie (With Yajirobe staying behind, though I've not got much else down for his overall role... XD).
Ur, wait, what?
Right. Piccolo.
I'm actually kinda expecting his fate to turn out more like the original (Egg Jr and such), though probably for unfounded reasons.

As for how Goku 'kills' him... Kamehameha, probably. Unless Roshi actually defeats him. Actually, I could see that working; the Mafuuba actually working, killing Roshi, sealing Piccolo again, only for him to be released in the sequel once the goodguys need extra firepower (or maybe the baddies would find him, I dunno. Since I kinda doubt the movie is going with my idea on how the Movie Mafuuba should work... XD).
Good grief, I did it again. T_T Sorry everyone.