I hope, although I have my doubts, that Frozen 3 won't be yet another story where the villainess is portrayed as a major threat throughout the entire movie, only for it to be revealed at the end that she's simply misunderstood and just needs understanding or acceptance. Or, even worse, that the real villain turns out to be some random character, while she's portrayed as the actual victim.
Although, I'm afraid it might end up being some kind of "what if" scenario, where the villainess represents what Elsa could have become if she had never had Anna in her life. Basically, without Anna, Elsa would have turned evil too, and by the end, they'll just show the villainess the power of "love," and suddenly everything will be fine. Essentially, a storyline similar to what we saw in Once Upon a Time.
Most importantly, there's no need for this villainess to have excessive screen time, or even worse, to become the center of the entire story and overshadow Anna and Elsa, just like what happened in Once Upon a Time.