I'm new, i'm not very good in english … but i will give it a shot ^^
My biggest concern is that the next arc* (maybe they have an adventure on the sea first.. who knows..) luffy, robin & chopper will be useless...
merman island is under the sea.. so my guess is those 3 will have a little adventure above the land.. (blackbeard/ace/whitebeard/shanks/ who knows...)
nami is waiting for the log with usopp/sogeking and are terified to death.
Franky who is now teaming up with sanji are having a conflict with some mermans (dunno :p) Zoro on the other hand is finding himself a sword that has that material seastone (you know .. all DF-users are weak from)
(i don't actually think because zoro would be overpowered so let's say a new sword) nami and usopp having an unexpected visitor namely arlong himself and is seeking for revenge... sanji and franky teams up against the big boss.
Zoro finds the star dude, the mermaid and that 8-sword user (you know) and the star will join the crew!
xD sorry maybe there are more threads like this you can count but ach heck...