I honestly want them to stop with this mystery bad guy thing they are doing in their shows so far. The problem of why it probably isn't something else is that you can't introduce THE villain in the final episode like that unless it is purely for set up into the movies or whatever. That's possible I guess, but it's not satisfying either for the show itself.
For me introducing a brand new person like that in the final episode is boring and doesn't let the TV show be its own thing very well. We will see.
There is not even a good candidate for a villain that has been already introduced.
-Renslayer is unlikely since in Sylvie's flashback she is a Hunter and not even a judge yet and she seems to be working for whoever is in charge, whether she actually knows who it is or not.
-It could be a variant Mobius since they've teased that another Mobius may be working with Renslayer, but he doesn't seem to have the motivation to have only one Mobius working there, or to set up the TVA/sacred timeline, or to go after a bunch of Lokis
-Maybe Classic Loki just needed the help of competent Lokis that weren't backstabbing each other to get rid of Alioth and that is why he couldn't get past that end of the timeline. He has been around the longest after all and probably knows a thing or two. The thing is that his backstory sounds legit and he doesn't seem all that bad. He would have also had to have been betrayed by Renslayer to be pruned to the end of the timeline, which doesn't sound like much of a mastermind thing to happen.
-President Loki seems pretty evil but not very competent and he already had his plans fail almost as soon as we met him
-Sylvie seems like she has had her life taken from her and her quest to stop the TVA seems legit
-Maybe it's Miss Minutes, but if that's the case, I have no idea who she is or what her goal is. It'd be a pretty left-field reveal, despite her being a little sus.
That makes it seem like even if it's another variant Loki, who seems like the most likely candidate at this point, it's totally a brand new villain they are introducing in this final episode. We may be familiar with Loki variants in general, and another variant may be very similar to Lokis we already know, but there will have to be something about this Loki that makes them different and we'll have to learn what that is and their backstory, and if that's the case, I think basically anything is on the table. He Who Remains or Kang/Immortus would be the same deal; you are going to have to explain what is going on.