Think its clear that both Fujitora and Ryokugyu wont be on the bad guys side until the end of the story, Oda have already given hints on that in the SBS.
Ryokugyu comming to Wano is pretty much like Garp going to W-7, to make for future setups, to give info dump about the world and force the SH to move on to the next Island.
If he is comming alone its clear that wont be a WG take over of Wano, Momo chosing to not open the borders yet is also to prevent this. Believe most likely when he get to Wano Momo will be the one to meet with him, with the samurais of Wano in his back, as a way to show to the WG that Wano is again under the Kozuki protection.
Also liked the ideia of he doing something to make trees grow all over Wano to show his powers. A way to make he leave Wano would be for he to be part of Sword and meet with Drake, that could tell him about the CP0 guys doing deals with Kaido.