that sasuke picture reminds of this (Alucard's form) :
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anyways, sasuke won't be the final villain in the sense of "the biggest threat to NV", he will be more like naruto's final opponent (to resolve personal issues).
i mean if sasuke doesn't have a juubi on his side, then the armies of shinobi's and all the kage level ninja's (including naruto) will just mob the floor with him.
i think it's kind of obvious how it will go down, naruto will choose to face him ALONE (might ask the rest to not interfere), or after they finish all of the fighting (stop obito) sasuke will run away and meet naruto later in the same location hashirama and madara fought for the final showdown.
one thing is for sure, sasuke is NOT a good guy, he plans to change the system, kill the bijuu's (including the one inside naruto) and wants to become hokage to enforce what he thinks should be "the shinobi world" so he is not on naruto's side.