Imho imu is the final villain. And I think that bb is also crucial, because imu, bb(ancestor) and joyboy have some connection. Most probably bb ancestor's will remained in the yami fruit, just like nika's.
We have lily, who "betrayed" imu and the other 20 kings, so we have bb ancestor, who most probably betrayed the d's, as bb was shown always to have opposite feats than other d's.
Dude thought that he should become the king of the world, betrayed nika in the most crucial time, which is why he lost and later got his ass kicked by imu.
I wouldn't be surprised if nika shared some sake with bb and imu to become brothers, only to be backstabbed by both of them lol (in contrast to luffy ace and sabo).
So yeah, final villain should be imu.