It would be difficult to justify being a villain in a traditional sense, but I could see them doing something where you're being painted as one by the antagonists. Something like with 2.5's ending, but have that be near the start of a new expansion, and go from there.

The WoL could be imprisoned near the start, and escape with the help of another convict, or several. Could have them become new party members. It would be more interesting if some of said convicts may have actually done some stuff to deserve being there.. I would appreciate having more varied, dissenting voices in the WoL's party, or just those with different moral compasses or motivations. I love the Scions, but they're nearly always of one mind for how to resolve certain crises.

A vague example would be having the opportunity to kill one of the lesser antagonists, but the WoL lets him go because they don't see him as a threat. One of your new convict companions argues with you about it and insists it's a bad idea. Maybe it comes back to bite you all later, and he could berate your decision way more vehemently, maybe even fights you over it. Interpersonal conflicts!

I do hope a more mature tone is something we can get back though in general. It's not about having a quota of dark THINGS happen during the MSQ, but how it's all told. We have an expansion that simultaneously has
a dead baby cave and a 4kids rubber bullet duel
and it clashes too hard. I'm fine with side quests having sillier, lighter things, but we need a more consistent tone for the MSQ.