Feel free to add more responses!! i.e. if there's any Pokémon you think might gain a new Mega Evolution upon the release of Pokémon Legends: Z-A, an upcoming game which is confirmed to have Megas.
If that new Mega Evolution is present on the game's release day (or is announced before then), that option will resolve YES.
For any "...again" options, that Pokémon would have to receive a NEW additional Mega Evolution (e.g. a hypothetical "Mega Charizard Z", for instance).
@Najawin - Ooh, what a fun answer! :D I don't think Floette (the middle evolution) will get a Mega, but I do think the "Eternal Flower" form of Floette will become obtainable. (But, unless something gets labelled as a Mega Evolution, it'll resolve NO.)
@PaintspotInfez That's my guess as well, but if there's ever a middle form that does get a mega, it's probably this.