I think its going to happen the way oda has things set up so far, its looking like we may get to see chopper and luffy fighting together (or near each at the same time). But if this does happen, and the way things are setup, especially with all the talk of zoan fruits by Lucci lately, and how powerful they make the user physically, and since chopper is a zoan himself…
I can totally seeing Lucci eating a rumble ball in his fight against luffy. Especially if luffy shows up late and chopper had gotten there first (which is probably going to happen given luffy's sense of direction and choppers luck, eg. skypia). Even if chopper ends up arriving at a similar time and fights spandam, i still think lucci is gonna get one of his rumble balls somehow and eat it, or at least i wanna see a zoan really bad besides chopper eat one heh.
The only other option i see for chopper right now is him fighting with sogeking, but i really hope oda wouldnt repeat something he did in alabasta... then again that could leave it open for jyabura to eat a rumble ball, but i would still rather see lucci do it if its gonna happen.