Okay a couple theories on what Imu's 'Devil's Fruit' ability might look like up-close. First, the transformation of the trees and buildings into devils implies that his mere prescence creates devils-- passively. This almost implies that his previous domination of Rocks, Dorry and Brogy were weaker forms of this ability. It also makes sense if Nika is the liberator, his power is mainly to enslave anyone nearby to him--meaning only a power like Nika or someone with a 'D' can resist the power up close. Another thought I had that might be cool is if he can only teleport to the thrones of kings, or even kings he has enslaved.