Is she really? As of the end of AatW she's basically a good guy.
This is really the biggest problem with a Thunderbolts/Dark Avengers concept based on where the MCU currently is. I can come up with a list of names that could be on it, but it doesn't feel much like a natural, workable list.
Ghost is a good guy. Vulture is bordering on repentant. Zemo is probably even less repentant than before after he sees the Avengers name recover and wants to finish the job of ruining them, but he also has no powers and would really make more sense as a 'how do we destroy the avengers?' consultant.
Any one of these could work, I think, as the 'exception' on the team, and maybe even two together would be fine, but all three of them feels kind of weird.
Mordo is unconcerned with non-sorceror stuff and too powerful for the govt. to find/control him. Shocker, Tinkerer, Mysterio and/or Scorpion could work, but then you have half the team packed with Spidey villains and it starts to feel like the Sinister Six. Grandmaster is way too powerful. Red Skull is on an alien planet (and Hugo Weaving isn't interested). Loki is dead (and too powerful, anyway).
I suppose they could theoretically find Yellowjacket in the quantum realm, disfigured and pissed off, or rebuild Ultron with some programming tweaks to force him to obey, or reveal there was a secret back-up of Arnim Zola and a prototype robot body. That would make things a bit more balanced.
And there are still other films coming out. Maybe Captain Marvel will introduce someone who's usable. Or Black Widow. Other than that, it seems such a film will kind of have to introduce quite a few characters brand new.