With the release of Mega Sceptile and Mega Swampert, the good old trademark list seems even more legitimate. However, ignoring the ones that are present there, what Mega Evolutions might happen?
A stupid route would be to look at the direct counterparts of existing Megas, and other trademarks. Which would be;
-The Pikaclones, as Plusle and Minun has been recently trademarked.
-Sableye, as Mawile has a Mega.
-Gallade, as Gardevoir has a Mega.
-Maybe Dusknoir, even though Dusknoir already has a similar BST to Banette.
-Jirachi, the counterpart to Diancie. Likewise, Manaphy, Mew, Celebi and Victini, as well as potentially Shaymin and Meloetta(even though the latter two already have Megas)
-Metagross, Salamence, Hydreigon, Dragonite and maybe Goodra, as Tyranitar and Garchomp have Megas.
-Illumise, because Volbeat is trademarked.
-Luxray, Raichu and Zebstrika, due to Ampharos and Manectric's Megas.
-The myriad of Fossil Pokémon, due to Aerodactyl
-The other starters, as they are all trademarked, marketable, and because we now have two sets of starters with Megas.
-Milotic, as Gyarados has a Mega.
-Golem, Gigalith, Conkeldurr and Machamp, as Alakazam and Gengar have Megas.
-Hippowdon(and Politoed and Ninetales), as Abomasnow and Tyranitar have Megas.
-Zoroark, as Lucario and Absol have Megas.
-Mightyena(and Liepard), as Houndoom has a Mega.
-Mr. Mime, as Jynx is trademarked.
-Ludicolo, Shiftry, Seviper, Lunatone, Solrock, Zangoose, Seviper, as they are version exclusives for R/S.
-Rhyperior, as Aggron has a Mega.
-Gothitelle and Reuniclus, as Gardevoir and Alakazam have Megas.
Another speculation that came out of /vp/ is that all important trainers might Mega-Evolve their aces in their rematches(and the team leaders and Steven can have their megas in the climatic matches). Going by this, we would have:
Roxanne: Mega Probopass(?)
Brawly: Mega Hariyama
Wattson: Mega Manectric(Which already exists lol)
Flannery: Mega Torkoal
Norman: Mega Slaking(Oh the horror)
Winona: Mega Altaria
Tate and Liza: Mega Solrock and Mega Lunatone
Wallace: Mega Milotic
Juan: Mega Kingdra
Sidney: Mega Absol(Which already exists lol)
Phoebe: Mega Dusknoir(?)
Glacia: Mega Walrein
Drake: Mega Salamence
Steven: Mega Metagross
Maxie: Mega Camerupt
Archie: Mega Sharpedo
Wally: Mega Gardevoir(Which already exists)/Mega Gallade(Since Wally's Ralts is always Male)
Brendan/May: Mega Swellow/Tropius(The former the permenant team member for their last rematch in R/S, the latter fills the same role for Emerald)
And for the lulz;
Gabby and Ty: Mega Exploud and Mega Magnezone
Anabel: Mega Snorlax(?)
Brandon: Mega Regi trio and/or Mega Bird trio
Spenser: Mega Slaking(Which is already Norman's signature)
Lucy: Mega Seviper
Tucker: Mega Swampert(Which now exists)
Greta: Mega Umbreon(??)