I honestly would love a villain like Talpa from the classic Anime Ronin Warriors. I really like the idea of a villain that's deeply tied to the morphin grid and knows a lot about it even more than Zordon. Granted this could apply to Dark Specter but I enjoy the idea of a villain showing up capturing the humans almost instantly and for his generals to not be just evil psycho or dark rangers but humans with regular ranger powers.

I mean I think that'd be more twisted to encounter rangers who are not just out to defeat the rangers but to corrupt them as well. Like pushing them in a battle until they give in to their more negative emotions. Especially if it was revealed by the big bad and his generals that their ranger powers would slowly draw out and guide them to their true nature. Doing things like separating them trapping them in illusions and tricking them into destroying parts of their hometown and then saying "Do you still think you protect this city? Soon you'll serve me."

I think a villain that messes with rangers like that would be interesting especially if the Rangers were fighting an uphill battle from the start.