Not sure why you're picking specifically from the Hoenn dex when Lopunny, Audino and Slowbro make it clear to me that GF are going to give Megas to whatever the heck they wantYou 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).
Funnily enough, Light Metal will be the better ability for it in the same way Light Metal is Scizor's better ability now - you're going to mega evolve right away so might as well reduce damage from a few minor moves.True, true, which I suppose that's why they're giving away a Shiny Beldum since Metagross's Hidden Ability is Light Metal... yeah, I'd go for the Shiny Metagross myself.
... Actually, no wait, clear body preventing intimidate... j-just ignore everything I say I'm dumb