Pokemon Legends Z-A is the game where we finally get to see more Mega Evolutions, after Gen VII only featured those that could do so already, and no game after Gen VII up until now had them. No new Mega Evolutions have been confirmed just yet, but with whatever there ends up being, it's important that the right kind of choices are made since these might likely be put back to rest come Gen X. This means that with whatever Pokemon gains itself a Mega Evolution in Legends Z-A, any Pokemon that's a counterpart to it will have to do so as well, for instance with Turtonator if Drampa does so, and Clawitzer if Dragalgae does so, and Toxicroak if Scrafty does so, and so forth. Otherwise it'll be like Pokemon such as Alakazam and Milotic and Duskinor, which are Pokemon that are counterparts to those which already have Mega Evolutions, a sign that they are destined to have Mega Evolutions or something similar themselves at some point, by something similar I'm referring to how Butterfree as Beedrill's counterpart has a Gigantamax Form (though it can only be used in Galar and has it's differences). 

Good luck I say either way with whatever Pokemon get given Mega Evolutions for the first time in Legends Z-A, what matters is that it follows the drill of having atleast one from every gen and that the right choices are made. Although if Gen X's new region does be one to have new mega evolutions as well as a new gimmick through means of celebrating the series' 30th Anniversary, all can be assured with what doesn't get given a Mega Evolution in Legends Z-A, these are not something that can only ever be doable in Kalos, which now has had it's time to revisit come and go (albeit only with how it was in the past and not also in the present day for some reason).