I was thinking of like:
- Pre evolution of Pokemon that don't evolve and hasn't evolved (like Lapras or Tauros or Miltank or Kangaskhan)
- Pre evolution of Pokemon that evolve once and hasn't evolved (like Scyther or Houndour)
- Pre evolution of Pokemon that has evolved once and can't evolve (like Houndoom or Scizor or Volcarona)
- Evolution of Pokemon that don't evolve and hasn't evolved (Like Lapras or Tauros or Miltank or Kangaskhan)
- Evolution (split) of Pokemon that can evolve (like Poliwag, Scyther, Eevee, Riolu, Pawniard, etc.)
- Mega Evolution / Primal Reversion of Pokemon that can't mega evolve (FLYGON, MILOTIC, BUTTERFREE I'm looking at you Gamefreak)
- Mega Evolution (alternate) / Primal Reversion of Pokemon that already can mega evolve (think something along the line of Mega Evolution X and Mega Evolution Y)

Pokemon that only evolved once are treated like they evolve twice for completion status
Pokemon that evolves twice don't change quota, just has different name / aesthetics
The same procedure to Mega Evolving would apply - you'd just need to buy Fakite (TM) or whatever name you want.
(To stop at the pre-evolution or a middle stage you'd still need an Everstone though)

Mega Mega wouldn't really work because you'd need a Pokemon to hold a Mega Stone while holding another Mega Stone.