Idk why but I've actually never really considered Usopps weapon getting a fruit that could be cool.Luffy - keeps the gomu gomu no mi - need i say more for him haha
Sanji - kizaru's devil fruit, also needs advanced arnament&observation haki
Zoro - if he gets one then ....Mr1's devil fruit but still uses santuryou style. Needs to get advanced arnament haki and advanced observation haki
Jimbei - being a Fishman with mastery of Fishman Karate isn't likely to give up the ability to swim considering how many times it's saved Luffy already. Already has haki just more practice i guess.
Brook - keeps his, observation and arnament haki.
Chopper - keeps his, plus some arnament and observation haki.
Nami - Monets Snow-Snow fruit, plus arnament haki would allow her to attack other devil fruit users
Robin - Alvida's devil fruit, think about it, whereever she spurs hers features someone tries to grab them they slip haha. Also, needs observation and arnament haki.
Usopp - I dont see him eating one but his kabuto might. We've seen things eating devil fruits before and one on the crew would be pretty cool. Other than this, if he has arnament and observation haki it would suit him well as a sniper.
Franky- Doesn't need one, just needs basic observation and arnament haki.