While I think having a new antagonist for season 5 would be interesting, I feel like the Duffer Brothers should focus on the antagonists they left us with in season 4. I think introducing a new antagonist would be a little risky, given that season 5 will be the last season, and a lot already needs to be resolved without introducing a new antagonist.
I think the Duffers have basically said that there will be no new major characters (antagonist or protagonist) in season 5. They already have enough story to wrap up in 8 episodes, and don't need to be complicating matters further.
PLZ No new antagonists! It's the last season: If they introduce some new antagonist, it just hast to be weak, or die because of a weird reason. They shouldn't play that card to guarantee the season's quality. It don't has to be like: Wow, what an ending... We killed a boring and just lame monster, that the duffers just introduced to introduce some new stuff for the last season. WHY should they "get rid of" Vecna? They built him up!
Protagonists no, it would be so unnecessary. I like the idea of having a "new" but technically already existing antagonist though, like the true form of the Mind Flayer mentioned here. Someone or something that has been there since the beginning that we just haven't noticed at all.
