Apart from few pokemon, PMU has almost all features of gen VI. I would say the last feature to make it a complete gen VI game is to add Mega Evolution. I do understand that the mechanic and duration of evolution could be different in PMU than the core games or PMD… And knowing how difficult it is to hunt rare items in this game, mega stones might be super difficult to get.
I strongly suggest that mega evolution is added to the game though, as it will make it more interesting, give use to new unused Pokemon and provide useful bulk in tough dungeons. I think mega evolution in PMU should work more like the core series, giving just a 100 base stat increase, and alter typing/abilities. I found the awakened status in PMD pretty OP and broken.
In PMU, perhaps mega evolution could be achieved by getting the Key Stone as reward from a “mega evolution dungeon”, much like the tablet. And then either randomly find mega stones in sealed rooms/boxes in the same or a different dungeon. There could be either stones specific for each pokemon, or we could also have a generic mega stone for all species.
Of course these are just ideas, but mega evolution is important for a complete game experience imo!