I feel like they could have just made it so all pokemon can get a mega boost, but they just get a basic stat buffs and an aura if you really want every pokemon to be capable of using the gimmick. Like have mage stone just called megaite or something that's universal, while stones exclusive to a singular pokemon are stronger and undergo physical changes and type changes.
Really this feels like throwing the baby with bath water approach, when they simply could have tried to refine what they had.
Also held items are fine you are trading a pokemon's utility for more power in a sense you should question if having mega is truly needed.
The best megas in my opinion are the ones that don't want mega evolve instantly into battle, Soclipede has speed boost, and when it megas it loses speed, so to make up for that you'd want to hold off a bit to gain those speed boosts so the stat cut isn't as brutal.
I feel like that's conceptually more engaging.