Still pretty hopeful for my Prankster Mega Masquerain though. Come on Gamefreak let it be Masquerain to be the token Hoenn Bug Mega. Kudasai! :P
You bring up an interesting point. Pages ago someone posted ALL the Mega Evolutions by their type, but I don't think anyone posted just the Hoenn Mega Evolutions by their type. We know that every type has at least one representation with Kalos' Mega Evolutions, let's see if the same is true for Hoenn's:
Normal: 2 (Lopunny & Audino)
Fighting: 3 (Blaziken, Lopunny, & Gallade)
Flying: 1 (Salamence)
Poison: 0
Ground: 2 (Swampert & Camerupt)
Rock: 1 (Diancie)
Bug: 0
Ghost: 1 (Sableye)
Steel: 1 (Metagross)
Fire: 2 (Blaziken & Camerupt)
Water: 3 (Slowbro, Swampert, & Sharpedo)
Grass: 1 (Sceptile)
Electric: 0
Psychic: 5 (Slowbro, Gallade, Metagross, Latios, & Latias)
Ice: 0
Dragon: 5 (Sceptile, Altaria, Salamence, Latios, & Latias)
Dark: 2 (Sableye & Sherpedo)
Fairy: 3 (Altaria, Audino, & Diancie)
So we still need a Poison-, Bug-, Electric-, and Ice-type. Looking through the old Hoenn Dex, we have the following options for each (of course, possibly any Pokemon could be chosen since a Mega Evolution can change type, but that's impossible to predict. Besides that, So far at least one type had a Pokemon that was previously that type before Mega evolving):
Poison: Dustox, Crobat, Tentacruel, Vileplume, Roserade (Roselia is in the Hoenn Dex), Swalot, Muk, Weezing & Seviper
Bug: Beautifly, Dustox, Masquerain, Ninjask, Shedinja, Volbeat, Illumise, & Armaldo
Electric: Plusle, Minun, Magnezone (Magnemite family is in the Hoenn Dex), Electrode, Raichu, & Lanturn
Ice: Glalie, Froslass (Snorunt family is in the Hoenn Dex), Walrein, Regice
If I had to pick, I'd choose Crobat (Roserade a close second), Masquerain (though Mega Ninjask would be hilarious), Raichu (so WHY hasn't it been given a Mega Evolution yet?), and Walrein (Hoenn sucked for Ice-types, but I'm choosing Walrein since Glalie and Froslass are a pair so it wouldn't seem right to give one a Mega Evolution over the other).