I feel like in the season finale, 11 kills Vecna/Henry/01 and then absorbs his powers. Military comes in and tries to kill her but end up killing Mike instead because he shoves her out of the way or something. 11 goes crazy and transforms into a monster which leads into the final season. OR maybe Mike lives and 11 still becomes evil, I feel like that's just going to happen.
I do think a villain turn makes sense for El but I don't think the spirit of the show would allow her to remain a villain for an extended period of time. If she's going to go bad, it's going to be a relatively contained affair, I just can't see her being the big bad for S5.