I wouldn't be surprised if this has been suggested, but here goes: Why not make Legendary pokemon PvP Events? It could be made so that once a legendary is encountered and captured a system message goes out about who caught the legendary. The player should be required to accept duel requests and lose the pokemon if the duel is lost. All pokemon including the legendary should be healed and cured of status effects before the next duel. If the player logs out or attempts to store the pokemon it should be released instead. Obviously only one of each should be available at a time.