While I do think many Kalos Pokemon should get Megas, I also wouldn't mind to see other gens get new Megas too, like each gen get at least one new Mega. While Kalos mons lacking Mega was an issue, the most prominent issue with Mega was not many mundane Pokemon got Mega while probably too few popular/good Pokemon got it. And that's not covering the other issues Mega Evolution had like, since only one Pokemon can Mega Evolve, and all Mega got a +100 BST no matter their stats, it means Pokemon which were already good that got a Mega were the only ones used (or the ones which got OP Abilities). I like Mega Evolution but I agree it's a mechanic which needs some improvements and hopefully they'll at least try addressing the issues.
Considering 100% is already using 5 cores I don't think we'll see a 200%... BUT I wouldn't write off new forms. I joked about a 75% form, but honestly that's probably the only place they can go. If I were designing it, I would actually make two 75% forms and have them be Zygarde either mimicking or more directly countering Xerneas and Yveltal; and justify it by saying in-lore the 75% forms have 3 cores operating it.
Also I don't see Zygarde getting a Mega but can for Xerneas and Zygarde Yveltal, maybe justifying the reason for two new Zygarde forms.