Yup, the playbook determines the XP triggers, so it's all about how you want to play the villain. I think I'm also pretty sure that the veteran abilities thing needs to be separate from the powers thing. That said, I'm still waffling on exactly how to realize powers.
I took a hard look at City of Mist and there are some great ideas there. I really like their use of tags for powers, and I really like how power tags can either stack for effect or go wide if you want diverse abilities. I'm still not sure how I want to graft tags onto the die pool system though. It works very intuitively in PbtA's 2d6 system, and I'd like to retain that feel but combined with the die pool mechanic native to BitD. Some of the moving parts to work other than just extra dice with are things like position, potency, scale and quality, plus additional effects from tags.
My initial thought is to attach a power to an action. Your power is Fire Blasts so you attach it to Scrap. You give it the Tag Red Hot, and say it adds 1d to ranged attack rolls. The XP or build cost would be the same as any other point (I think). You can spend additional points to add additional tags, so maybe Flaming Fists allows the +1d in melee, or Blast Focus adds potency to ranged. There should probably be a cap on straight die adds, maybe at +2, IDK.
What I would like is for those tags to also be more broadly useful in a narrative sense too. If the player thinks of a way that her Fire Blasts could help with a skill different than the one it's attached to she should be able to use there as well. That's the part I'm not sure how to handle. Maybe Blast Focus, from above, used outside it's keyed action (Scrap), simply adds a die. So our Fire Blaster, lets call her Volcano, asks if she can use Focused Blast to soften the padlock before she tries to bust it open. The GM says sure, take a die and roll Wreck. So instead of her normal 2d in Wreck she rolls three.