Deadeye
H(a)unting...
They're not going to introduce a bunch of Megas for already-very-useful species, so for there to be any introduction of new ones, we need to have a solid selection of weakened species with the design or popularity to justify the effort of designing these Megas.
I would like it if they did, because imho adding Mega/Primal evolutions for legendary Pokemon was a poor choice. If Game Freak let legendaries be legendaries with some air of mystery around them and focus on making those species strong that any trainer could be raising if they lived in Pokemon world, that would both highlight more what Pokemon is... or rather once was about; working together and bonding with your partners, not to mention having friendly battles where every Pokemon has potential to become strong! It could bring balance to wifi battles as well considering teams with legendaries usually win those with non-legendaries, even while some over used non-legendaries stand a better chance.
Generally I didn't like much about Z-moves, seems like a hasty attempt to bring balance for those Pokemon that don't mega evolve. I wouldn't be bothered if they were gone but I could tolerate them still existing in Ultra Sun/Moon if there was a significant purpose for them.. aside from just being a new cool feature. Wonder if the Z-moves have some vague connection to Zygarde which would be introduced in the next game? They've been stalling this much all the way from X/Y.