Torkoal was already done by
Dry Paratroopa, so hopefully he'll argue his case here.
My idea for Mega Torkoal would be Fire/Water with Scald, and turning it from a tortoise to a turtle. It already has the steam flavor handy, and will maintain its bulk with Filter. Shell Smash can be used if necessary, but Torkoal's awful speed does it no favors on that front.
Mega Tropius is begging for Grass/Dragon, and already has Dragon Dance, Outrage, and Dragon Pulse to boot. Leaf Blade/Outrage/Earthquake only misses Skarmory, Togekiss, Whimsicott, and Bronzong, so Dragon Dance is nice and viable, but it can also be tanky with Leech Seed if you'd like. Buffing Tropius into viability, beyond the stat increase, is new ability Filter (sorry two Filters happen to be in a row in this project), which aids its weaknesses to (takes big breath) Flying, Bug, Poison, Dragon, Fairy, and Ice (x4), and lets its nifty resistances to Water (x4), Grass (x4), Electric (x4), and Ground shine brighter.
Mega Wailord is getting slower and letting those defenses make that 170 HP get some actual work done. While its mediocre 90/90 attacking stats remain the same, Sheer Force oughtta help it run a nice Curse set (but sadly makes Scald stop burning, rats!). I'm also letting it learn Rock Slide for coverage and because this is a whale that can go into battle on land, so I'm sure it can make some rocks fall. The beauty is that, with lowered speed, that flinchrate for Waterfall and Rock Slide were worthless anyway!
Mega Walrein is also beefing up, but gets an Attack boost as well. Simple boosts, but effective ones, particularly when it also gets Icicle Crash (how did it not already have this?) and, more importantly, Ice Age, our new ability that turns all non-SE moves into Ice-type. Given Walrein resists Ice x4, that means our walrus is suddenly an incredible tank, despite weaknesses to Electric, Grass, Fighting, and Rock.