Even if it would be late, if I have to pick out of the Pokemon Ash currently has in reserve for his Mega Evolution, I'm gonna have to say Sceptile. No one likes Mega Heracross. I have not really even seen any mention of Mega Heracross and even then, it's only to say that it's subpar compared to the others. Which leaves Charizard and Sceptile. But Charizard has the ability to utilize two different mega evolutions and since the special has showcased that the Pokemon wear special harnesses to hold their mega stones, they would really only be able to showcase one of them. Though it would be interesting if they added a rival with the opposing Mega stone...Anyways. Sceptile also has the added benefit of giving a nod towards ORAS, which a reference for them is probably going to squeeze itself in anyways so it would only make sense.
There is also Gible, but that would require a double evolution jump so I just can't see that happening.
Either way it goes though, I can definitely see there being problems with Sceptile and Charizard mega evolving. In both Charizard X (which seems to be the favored mega of the two, having been featured in two specials already) and Sceptile's case, they gain the dragon typing. A type that has been notable with their...independent nature and incredible power in the anime for a very long time.
If we're hoping to see one before the league though, I really have no idea what they would choose. If I counted right, there are 34 Mega Evolutions currently revealed. 5 are legendaries, 7 are Pokemon that Ash already has or has one of their evolutionary line, another four are HIGHLY unlikely (Blaziken, Swampert, Gardevoir, and Tyranitar) for various reasons. Aerodactyl is also unlikely since Ash already has two flying types on his team (or will in Hawlucha's case). Which leaves us with 17 choices. 17 Pokemon to pick from when it is likely to become iconic with the anime. A Pokemon which will be just as closely associated with Ash as Pikachu or Charizard. Which makes me think we won't be seeing one