Pokemon team explains how Mega Evolutions are decided

This translated Nintendo Dream interview comes from Nintendo Everything...

ND: Was there some kind of selection criteria for new Mega Evolutions?
Ohmori: There were some criteria, but we started by handling Pokemon that we wanted to Mega Evolve for the needs of the story. Like organization leaders’ Pokemon such as Maxie’s Camerupt and Archie’s Sharpedo. Others were chosen while considering things like popularity and battle balance.

ND: Do you create a Mega Evolution by making the design first? Or are there cases when you wanted some ability for a Pokemon and created a Mega Evolution from that?
Ohmori: There are many Mega Evolutions for which the design was a priority. It differs from creating a Pokemon from scratch as you expand the existent design.

Sugimori: However, not every Mega Evolution got its abilities from its design. There isn’t a fixed procedure and each case is different.

Morimoto: There are also cases when we wanted increase things like attack or speed with a Mega Evolution. Those were decided first and it affected the design.