All pokemon that are one-stage should get megaevolutions. I.E. Dunsparce, Miltank, Tauros, Stunfisk, Pachirisu, etc. Also I think they should give any pokemon that evolves with a stone a megaevolution. In the game the Prof mention that the megastones might be ancient evolution stones like fire, water, thunder, etc. So it could be likely that they affect pokemon that evolve via stones already. I.E. eeveelutions, poliwrath, arcanine, vileplume, etc.