In other mobile games I've played, the "path" to earning previously-limited stuff is often just another currency. The currency itself is earned through normal gameplay or through events--not all routes are always available at all times--but rarely has only one mechanic by which it can ever be earned (and, importantly, it can only be earned through gameplay, not purchased for real money).

Once a player has earned enough of that currency, which can take months sometimes, they can trade it for a previously-exclusive thing from an in-game shop.

This method has several benefits and an equal number of drawbacks.

Benefits:

• Theoretically allows completionists to get everything.

• Usually doesn't require playing any one particular mode or engaging in any one particular mechanic.

Drawbacks:

• The stock of the exchange shop is usually several months behind the game (and therefore the powercreep), so the things for sale there are often not useful for much beyond completionists. Almost never is an exclusive made available there immediately. "Powercreep" may not be quite as big a deal in PoGO as in many mobile games, but it's not nonexistent here either. (T-tar CD, anyone?)

• Given that the required currency is deliberately slow to earn and cannot be purchased, nobody can expect to be able to buy everything from the exchange. On the one hand, this still encourages participation in events when they're live, so it benefits the longterm health of the game. On the other hand, it means that many players still won't be able to get everything.