>>18672848M-Torterra would be pretty easy. Just make it bigger and bulkier. M-Infernape would probably gain more elements of Sun Wukong/Son Goku, its fiery head of hair would grow bigger and longer, it'd have a bit more armor, etcetera. As for M-Empoleon, this one I'm not too sure about, but it could probably gain some more armor.
The Kanto starters, for the most part, did it well enough too. They all just grew bigger and bulkier. Some Megas I will say are kind of ridiculous, like Blaziken and Tyranitar, as much as I love those two, are a little over the top in design. But I'm sure they could pull it off for more Pokemon.
