We've had Usopp, Sanji, Franky, and Jinbe leave their groups to join the crew, so it wouldn't be the first time. Oda could also have them stay behind in Wano/Zou to help with the rebuilding or have them go searching for more Poneglyphs since Law and Robin discussed Poneglyphs and the Will of D.
I don't think Law will full on join the crew, but I could see Luffy being an inversion of Rocks where he gets a bunch of monsters (Law, Kid, Sabo, Hancock, etc.) as major figures that blur the line between crew member and ally to follow him through his charisma but does good in the world where Rocks presumably just caused terror.