ok now is my turn
Luffy- obviusly haki and aparently telekinesis to comunicae with hancock while on her ropes
Nami- she didnt make the tempo so there is no way she can upgrade it but she can collect some materials to give to ussop so he can upgrade it, so the tempo can be better but not right now
the windknots was just like a childs game to help nami`s mood, if that guy can create a strong wind with just a rope then that is just a really little part of what he can do
nami will learn to control the weather, with or without the tempo
Robin- besides learning more of the void century and the 700 years ago era she could learn to improve her wings, during just 5-10 secs make them almost useless, maybe she could go back to GL only using them
Franky- he will become stronger tougher and will have new weapons
Chopper- he arrived on an island were birds control human, the bird seem not to talk human but chooper can talk bird so i think he will learn something about the birds, maybe it will help him to control his human states, maybe go arm, brain, etc point without the rumble ball and then they will help him to go back to GL, the chopper be able to fly idea could be too we all know that reinders descend from birds didnt we
but is cristmas so a flying reinder is not that imposible
Zoro- zoro will learn tenkai, why you ask cause the idea of tenkai was to made your body as tough as steel, the reason why he coulndt withstand the dmg after a while is cause his body was not though enough, perona might help using her bomb ghosts or also improve his Asura, it is an spirit technique and perona is a spirit girl afterall
Sanji- he will learn rankiaku from running over the island
Usopp- fighting against man-eating plants is tough so he will have to learn to defend himself,
also if there are plants there most be insects, so he might invent new weapons or items out of insects + an upgrade on kabuto, maybe an armor like the one of herakles
and of course he will learn to use the same shooting technique that he use to kill the man-eating plant
i dont think is haki but it might be
Brook- Brook will improve his sword skill at the same point of ryuma, lola lost her shadow for 2-3 years(need confirmation here) and she remebered nami, brook lost his for 5 in which time ryuma trained farther using brooks shadow, and ofcourse he will be able to improve his music combat skills