G
Who do we think will end up finishing One Piece? Oda clearly won't.
Mashima could do it given how many series he keeps putting out and how he seems to be an unstoppable machine plus quite a bit of Fairy Tail is ridiculously similar to One Piece, at least character design wise and such.
Maybe now Odas assistants can actually take the brunt of the workload as they should, if Oda is unable to do it anymore, since he won't let them do what he loves doing far too much /s.