@The:
I really can't see a One Piece GT happening…least not with the original characters. I mean, with DBGT it was easy: It started a new story based in the training where DBZ left off. Not the first time it happened in the series.
But with OP? The series will obviously end with them arriving at Raftel, and accomplishing their dreams. Thats a pretty defiite way od saying "Done"
Well I get your point but must disagree. Oda managed to design a really complex universe in One Piece and he might in fact go as long as he's pleased with. This is especially remarquable as the only "story" that were that deep constructed were usually books that were written in 10 + volumes.
Indeed Oda manged to create a whole new world while detailing its geography, history, political context and inserted the whole balance of powers (marine, pirate, shichibukai, WG) ruling it.
The only other stories that really featured so much details on their universe (while not beeing based on real history) were in fact probably star wars, but also Lord of the ring (which though i'm not very found was very complete and became the basis of lot of games, story, movies), The hitchhyker guide's to the galaxy (my personal favorite though quite fucked up) and Dune (which probably is one of the most complete universe ever coming from a human soul).
So while I would definitely consider the extension of One Piece by anybody other than Oda, one could argue that he set enough bases, rules and characters for his world to be used for additional stories, even efter raftel.