I think that it will depend on the reward. The better the reward,
The less side quest exclusive Pokemon available.

Hmm a "call a friend" thing might be interesting xD

EDIT: also they would need to be done in times lots between gyms. Eg there isn't muh time between rustboro and Dewford, but a fair bit between Dewford and Mauville, or Lavaridge and Petalburg. That way if it goes longer than expect, everyone else isn't waiting.