There's a good chance the next games to get remade (not saying that the next games will be remakes, but the next remake they make) will be 4th gen. With that in mind. I think these pokemon should get megas:
Torterra, Infernape, and Empoleon need mega evolutions, along with Bibarel, Staraptor, Luxray, Floatzel, Skuntank, Purugly, Vespiquen Toxicroak and Drapion, Bastiodon and Rampardos, Hippowdon, Gastrodon, Lumineon, Bronzong, Spiritomb, Glaceon and Leafeon along with ALL of the other cross-gen evolutions (Magnezone, Electivire, Froslass, etc) the Lake Trio, the Kanto Bird trio, Heatran, two mega forms for both forms of Shaymin, separate mega forms for ALL forms of Rotom, including the base form, Manaphy,
Also, since there were a few baby forms of older pokemon, I think their adult forms should get megas. This includes:
Chimecho, Sudowoodo, Mr. Mime, Blissey, Snorlax, and Mantine.
Sounds like a lot, huh? 62 new Mega-Evolutions. A man can dream.