Been thinking about it more and I'm a lot less confident that MP will regenerate automatically over time than I used to be. I feel like the precedence we have for that isn't very strong, the stuff that gradually generates on a spectrum only includes ROB's fuel and Wario waft, everything else is one (or, in the case of Robin, several) use, then wait till it fully regenerates, then get one more use. The things that do auto-regen are fairly special cases in that you usually won't be specifically stalling for them for long periods of time. Wario players will, but they still need to interact with the other player in order to get them into waft's kill range, and the charge time it takes for waft to be useful is long enough that it's unreasonable to exclusively stall the whole time you're waiting for it. We can't really have that slow of a charge time because we're too crippled without MP for that to be useful (and I'm pretty confident we will recharge in some meaningful way, otherwise sizzle would have to be absurd to justify spending 1/5 of our MP).
Really, it just seems like poor game design for our primary method of MP regen to be "just wait" because it will cause players to try and stall and avoid interacting with the opponent for extended periods of time, which is the opposite of what you want from a fighting game. I think a manual recharge method a la Inkling or recharging when dealing damage are both reasonable methods, I'm beginning to think the former is more likely, but we'll see.