I think Lotor is going to be our final villain.
At the start, he'll be good while hiding his true intentions. What they are? I don't know but hear me ouyt.
They might have him pilot red since he and Keith are currently both hot shots. Keith will pilot black if they're serious about Allura staying in the castle to operate the ship.
Lotor will help the paladins track down Haggar and Zarkon, kill them, and then truly earn their trust.
This might be the end of S3, S4 will be about Keith and everyone else finding Shiro since the great threat is over.
This season might be the romance one, Allura wants to spend more time with Keith and he's okay with that.
Meanwhile Lotor realizes that he might like Allura and wants her since galra's are possessive in their own way. At the end of S4 Shiro comes back, but Lotor reveals that he is their new enemy.