I'd make it so just for one gen you would have to actually get every Pokémon to win the game and try to satisfy Gamefreak's original vision of forcing everyone to trade.

Benchmarks
50: Pokémon registered: you are allowed to face the first gym
100 Pokémon registered: you are allowed to face the second gym
151 Pokémon registered: you are allowed to face the third gym
225 Pokémon registered: you are allowed to face the fourth gym
320 Pokémon registered: you are allowed to face the fifth gym
400 Pokémon registered: you are allowed to face sixth gym
500 Pokémon registered: you are allowed to face seventh gym
655 Pokémon registered: you are allowed to face the eighth gym
800 Pokémon registered you are allowed to complete an optional challenge that unlocks transfer.
Full Dex including mythical and Legendary Pokémon: can challenge the Elite 4

If you have a legendary/mythical in Pokémon your party you can unlock a portal to face that same Pokémon once (you get one random freebie).
You have 1-5 daily challenges that let's you get a random regular foreign Pokémon meaning you have to trade alot as the regional dex excluding legendary and mythicals would be only 450 or so.