I think DR Luffy could be the most interesting story wise actually, and the SHP not abandoning their captain like the Rocks pirates has great potential. DR Loki is kind of forced, because the SHP and even the audience isn't really that connected to Loki.
But it's definitely still Imu as the villain of the arc, he's too tied to all the tragedy in the flashback with Harald and stuff. It would be like if Fujitora took over as the main villain of Dressrosa, or the Big Mom Pirates at FMI. It's always the character who did evil shit in the long flashback.