I think a "Voodoo Champion" is not something likely to be released, for a couple of reasons.
Cultural Concerns:
Voodoo is a real-world religion, so designing a champion around it is not something to be attempted carelessly, especially if you take into account that most of the concepts associated to Voodoo in popular culture are either comedic, inaccurate or downright racist.
Thematic Concerns:
What are the most recognizable aspects of Voodoo in popular culture?
- Raising the dead as zombies.
- Controlling or enslaving the souls of the dead.
- Manipulation of other people's bodies via the use of 'voodoo dolls'.
A Voodoo champion would probably need to hit at least one of those in order to be appropriately recognized as such, yet there are already champions in LoL that occupy the first two spaces.
Mordekaiser's ultimate delivers on the fantasy of enslaving your opponent's soul very well, and the ability is such an iconic piece of his kit that I don't see Riot removing it from him (nor bestowing it on two champions at the same time).
Yorick's entire gimmick consists on raising the dead to do his bidding, making very unlikely for us to see a second zombiemancer anytime soon.
Gameplay Concerns:
The last thematic space that a potential Voodoo champion could use would then be controlling a living enemy's body, but I don't think that is a design space that will be executed upon in the near future.
