Agreed. It all depends on what Mattel has planned. The only thing we know for certain is that, like Playmates, they'll be handling merch for whatever is new in the world of TMNT. Since they do collector-focused toys for other brands, I would expect they have plans to do that here as well, but is that a Masterverse take on the Mutant Mayhem designs? I guess it's possible.
My assumption based on absolutely nothing but my own gut, I would guess the overseas licenses are safe. Mattel probably won't impact JoyToy, Mety, and whoever else is making turtles for the Asian market in their own style. Super7, assuming they're happy doing 2003 designs, might be okay too. I don't see Mattel wanting to get into that playground. Mondo is doing 1:6 so they're probably safe and Mezco's price tag also probably means they're likely going to be able to continue if they so choose to. If anyone is in trouble I would guess it's NECA. The most appealing part of the franchise is still '87 and I wouldn't be surprised if Mattel wants to do something that at least evokes those classic looks even if they're not going for a "pulled right from the show" look. NECA's product would be competition and who pays for a master license for a property to compete against someone with a cheaper one? Archie may be too stylistically close as well for that line to continue. Maybe they keep things like Fortnite, Last Ronin, and Mirage but I don't know how much life is left in them? The 2012 line seems like the most obvious pivot for them, but I don't know how well that is being received. It might not be worth it to them to re-up on the license if that's all they have as I'm sure it's getting more expensive now that it's under new ownership.