Along with the prior discussion, I had postulated that it might take something new to bring it back to prominence and provoke a good response. Now, that something has become clear: that would be Pokémon Legends: Z-A, which takes the setting of Lumiose City as the heart of Kalos, the region that heralded Mega Evolution in the first place. In fact, new Pokémon with this evolutionary form have been revealed, from Mega Dragonite during the latest Pokémon Presents to those for Victreebel, Hawlucha, and Malamar in various (and quirky) social media announcements, and just now through a "final trailer" for other expected and unexpected species. Much, much more will become clear(er) when the game gets released next month, with this cavalcade heralding that clarity.
Then there is the TCG (physical as well as Live) with the coming Mega Evolution expansion. Some of the featured forms will certainly be for the "first batch" species that fans already know and recognize, and players will need to deal with them as "ex" cards that evolve from their earlier stage forms. Obviously, there was some planning for this to coincide with the Legends game as above, and that works out well for the momentum of that game while bolstering the TCG itself - allowing it to "take part" in the parade, in a way.
Meanwhile, there has been word that over the course of the remaining part of the year, Pokémon Unite will finally implement Mega Evolution for Pokémon other than Mewtwo (that has been the pioneer and yet still the only one as such even now). They'll be a sight to see when they finally appear, and their "ground rules" may also become evident at that point, but it is clear that Pokémon Unite has offered its contributions to the parade.
Of course, Pokémon Go will take everything into account. In fact, nearly all of the "first batch" species now have their forms in the game, or will eventually. That means the "second batch" from the new Legends game will follow suit, whether it's during this rather "form-changing" season or all the other ones that will come afterwards.
From its beginnings in the sixth-generation games, Mega Evolution truly has had its ups and downs. The momentum of the main series games that featured them is one upward moment, followed by the almost immediate downward one of their nearly becoming deprecated. Then there were just sporadic inclusions in other facets, before the current almost full resurgence with a "parade" heralded by a game that promises to be sensational. Like any parade, at least some excitement is to be had as its participants appear, and with the ones above for Mega Evolution, fans will definitely be set for a good time as they watch the "parade" pass by.
One year ago: On Half Decks and Full Decks
Two years ago: Spoiler Alert
Three years ago: Longing for Pokémon Donuts
Four years ago: The Fate of My Local Pokémon Hangouts
Five years ago: Competitor Ranks of the Silph League Arena
Six years ago: Personal Branding
Seven years ago: To Judge and Be Judged
Eight years ago: Catering to Old and New Players