Similarly, a debate that has been brought up constantly is if the morally shifting Charmcaster in later shows was on course to redemption as well or not.
Welp, I got in contact with Duncan Rouleau on Facebook and he set the record straight:
So no, Kevin and Charmcaster were never to remain villains. They were created as rivals/foils to Ben and Gwen who would gradually become more anti-heroic, and yes, Charmcaster would have joined Gwen's side at Friedkin University had that spin-off been made.
Now, I'm sure Man of Action would have done things differently (neither of them would "join the team" full-on like Kevin in UAF and Charm in the proposed Gwen spin-off, and Kevin in particular would be radically different as an anti-hero, keeping his old powers and personality. In fact, he's clearly a deliberate homage to Venom from Spider-Man: psychopathic evil counterpart to the hero who has the same powers but looks like a big monster and blames the hero for ruining his life rather than take responsibility for his own actions. Venom's style of anti-heroism would have been Kevin's style too.) But nevertheless, it's good to know that both characters were following the development path intended for them, even if you don't necessarily agree with the way that path was written over the course of the shows.
I, for one, am looking even more forward to inevitably seeing them in the reboot now.