There's no need for any incentives, the only thing SE needs to do is to block players from forming a party when not on their home DC. Players can still play with friends because the friend that they're visiting can form the party. If players cannot form a party on a DC they traveled to, most players will likely get tired of waiting for Aether natives to create a party that they can join, so they'll go back home to their own DC and make their own party.