The only way Dressrosa would become Luffy's turf is if Doflamingo's cruelty is made known to the broad public as Doflamingo is quite popular there. If Luffy would just overthrow him for Rebecca, they people would just hate him and it's really not in Luffy's nature to claim a territory in such a situation.
Also how could she possible become a Tenryuubito? The Celestial Dragons are the descendants of the ones who founded the WG and she isn't a descendant of them.
FI is pretty much already Luffy's territory as Big Mum gave and FI separated and Jinbei and Neptune were already talking about borrowing Luffy's flag. It's not really official, but in practice it's already Luffy's turf.
