I was thinking it could replace forage when a mega stone is held, and when G is pressed, the Pokemon would go through the mega evolution animation and turn into it's mega form, this would last for 5 minutes and then require a 1 minute cooldown. Returning the mega to it's pokeball or initiating a battle would end the Mega-Evolution, setting it on it's timer regardless of how much time was left.
I was also thinking that this could give some buffs to the pokemon, too.
For example, the two Mega-Zards. This could buff their flying speed, by 1.5x or even 2x. For Venusaur, it could buff land speed and allow it to plow through grass.
For Blastoise, it could buff swim speed by 1.5/2x.
The other megas, as they're introduced, can be given other abilities. Mega Scizor could have a mega-powerful cut that can chop down jungle trees.
The speed multiplier for those that have one could be a configured value for server owners worried that they may cause lag from generating too many chunks at once.
This is just a dumb idea, but I figured it'd look and work pretty cool and would make some sense with the pokemon series (See: Mega Latios/Latias in ORAS.)
Thanks for reading.