It would have been cool if Luffy effettictively died in Wano,and Nika would have taken his place,in his body. But,trough the following arcs,you could see glimpses of Luffy slowly coming back,because his soul cant be tamed by Nika. So in the end you would have had a Luffy/Nika co-existence in his body. Kinda like the Fred/Illyria situation in the tv show "Angel".