I don't really think it is a Great Unclean One and for one reason...
We can already see a part of the model sticking out; the Horn. It looks to me that the Horn isn't detailed very well, meaning that the model itself isn't very big.
I also don't think it is the new Beast of Nurgle, because we have seen it already and the lack of tendrils and tentacles on this teaser cross him out.
I think it is Nurgle Related, but I'm thinking less daemon, and more mortal. Has anyone thought Pestigor?
That or Nurgle Dwarf (I honestly really would like a Nurgle Dwarf).
We shall soon see I guess.