Just a few thoughts on recent comments.
I don't actually want to completely avoid a power list per se, I just want to avoid three pages of dense text describing every individual power in the known universe. I think once I have categories of power decided on, two or three example powers for each would be more than sufficient. I think the idea would be that some base mechanics are laid out, along with some examples, but after that its up to the player and GM to define the exact nature of the powers. That shouldn't be too much of a burden when its tag based, and I can steal some of the verbiage from CoM about overly broad tags and whatnot.
I think leaving a lot of this to the GM and players to kind of hash out and formalize is probably a good idea, and very in line with the FitD approach. Whether it's through Tags or Profiles or some other mechanic, I think just loosely defining and/or providing some examples to prompt players and GMs to come up with more is a good idea.
One thought I had along those lines is to add some dramatic tension to the game by having two mechanics that really pull the characters in different direction. Since we're talking villains, I think at least some of the XP triggers should be very personal and individual, and perhaps quite different from playbook to playbook. If, on the other hand, some of the mechanics for powers emphasized teamwork and cooperation to help mitigate or reduce stress, that could produce a nice push and pull between individual goals and team success. Especially if those system were purpose designed to work together to get the desired push and pull. I don't know if this exact example will provide the tension, but the more I think about the more I really want that tension in the game somewhere, somehow.
Tension like that seems like a good idea. Just an idea that occurred while reading....maybe place the selfishness of Batman's rogues at the heart of it. Maybe a specific XP Trigger rewards selfish behavior, but the mechanic that clears some stress is one that promotes teamwork. The PC has to decide which of these approaches to take, which reward to pursue. Self versus group.
Specifically on XP, I'll say I'm not sure. The baseline is obviously Blades but that may change a little once the details start to emerge. One thing I'm considering is either decoupling XP from the attributes and make it more personal beliefs and mission state oriented, or keeping the attribute XP and extending it to powers somehow. Powers could simply be treated as additional attributes, but since they are used in conjunction with actions, where do you put the XP for desperate actions, on the attribute or Power XP track? The option I'm currently looking at the hardest is to keep attribute XP the way it is, but tie power XP to personal belief and goal type stuff. Generally, your actions get better from desperate use, and your powers get better by following through on your core drives and motivations.
These seem like interesting ideas. Maybe allow some customization by giving each playbook a blank XP Trigger that they can choose? Maybe tie this to their origin/personal goals as you mention?
Band of Blades has a shorter Stress track to emphasize the mortality of your soldiers.
Maybe Gotham can have a longer Stress track? Alternatively, maybe allow 4 or 5 Trauma max instead? With a correspondingly longer list of Traumas? That would be fitting for our damaged supervillains.
Additional stress seems to be a possibility....but may tip too far toward PC strength. It'd probably take a good deal of testing to see how this would work.
Additional traumas seems like a less drastic change. Also, thinking about Batman's bad guys, I feel like having each starting with a Trauma already might make sense.
Yeah, a longer stress track might be in order for sure. I don't want things to get too gonzo though. I am going to include some mechanics for reducing stress, and its possible that not everything needs to work off stress either. Managing that resource is a key part of the game though, and I don't want to make too easy - players need to feel like they're being challenged. I think I need to lay out all the stress based mechanics side by each and see what it looks like. For example, with powers and tech, there may be less need to spend stress to resist in combat perhaps.
Another mechanic from CoM that I'm looking at is Burning Tags. You can burn a tag (that gets recovered in downtime) to add dice to a roll, or avoid consequences. So essentially giving up a portion of your power to not get dropped in the cacky. That could be something we can work with in addition to stress. At the very least I want to have some non-super power tags so that not everyone needs to be 'super'. There could be tech versions, and skill related tags. All sorts of options. They essentially work as free floating skills in CoM, which doesn't have named skills like BitD. That needs to change, but they certainly could work as specializations that add to skills in certain circumstances, much like we;ve discussed powers doing.
One thing that comes to mind here is how Blades doesn't differentiate between "powers" and "mundane" abilities. The Whisper can undoubtedly do some super stuff....but it's not really treated as different. So I'd think you could follow the same approach here. Mechanically, there's no need to treat the Joker's acid corsage differently than a punch from Solomon Grundy or a rifle shot from Deadshot or whatever. Narratively, yes they're different, but mechanically they can be the same, unless there's a compelling reason to highlight the difference.
I suppose the main difference is one of potential stress cost. I think that anything that would be considered a base attack (as they'd translate to Blades or S&V, at least)....a punch or a stab or a shot.....shouldn't cost stress. But tacking on an effect or an additional die should. So maybe Solomon Grundy has a Skirmish of 3d and that's what he rolls whenever he tries to smash someone with a fist. But he also has the ability "Strength of the Grave" (or some such) which allows him to add dice at the expense of Stress.
This happens to me a lot when I'm working on mechanics, I read something new, or different and get all ooooh, shiny about it. In the interests of keeping this project manageable I may avoid creating a ton of new bespoke mechanics. I think some of the powers stuff can be handled within the various playbooks, and something low-key on top of that. I'll check out Worlds in Peril and see what it has to offer. 
