The way things are going, it'd be Akainu.
Aramaki coming to Wano is to push the plot of SH's vs marines. It'd be used to explain what happened in the outside world while the SH's were fighting in Wano(especially the Vivi, Sabo, Hancock situation), showing off Luffy's threat lvl to Aramaki pushing the plot to lead into a conflict between the marines and the SHs, and thus ultimately leading to a Luffy vs Akainu confrontation
Another reason why I believe this is because the Blackbeard vs Shanks conflict didn't happen yet. That means imo that Oda is aiming at making it happen as a parallel to following conflict the SHs will have with the marines
Also if we look at the build up of this arc, Oda at multiple occasions was building up magma power with King's special move and especially Kaido whose ultimate attack against Luffy involved magma, and although he ultimately managed to overcome it with his haki, Oda still used that moment to show the lethality of magma. That suggest imo that he was preparing this match-up circumstance for when Luffy would actually face Akainu
But the conflict between SHs and marines would mostly serve to deal with the marines under Sakazaki's regime first imo, as a way for the "outward" face of the world government to be dissolved, and Imu, the 5 elders and Celestial Dragons to be exposed.
All of that potentially leading into a free for all between the SHs, Blackbeard with his goal to become King of the World, and Celestial Dragons, main antagonists to the Ds