Luffy - Facial hair, much more mature, ripped, will keep the straw hat and sandles. He'll development new haki/devil fruit abilities.
Zoro - Most likely will returned with visible scars, knowledge and improvement of haki, improved swordsmanship and new techniques.
Nami - Longer hair, new weather knowledge and an increased arsenal with new abilities.
Ussop - New hair, improved jungle/plant/survival knowledge and skills, improved arsenal of weapons and ripped.
Sanji - Incorporates Okama Kenpo into his fighting style, improved ability in martial arts, incorporates attack cuisine into his cooking/fighting repertoire and knowledge of haki.
Chopper - Incorporates new plants into medicines which could increase/extend any of his devil fruit abilities, more mature, taller and more developed antlers.
Robin - Connections with New World characters and the Revolutionary Army, increased knowledge of the lost history and current world events and will fu*k Dragon.
Franky - Improved arsenal and abilities, new hair, new tattoos, improved scientific knowledge.
Brook - Will learn to play and compose on guitar which he will incorporate into his fighting style/abilities.