How about this?
As we all know there is ten continents. Essentially for it to be over for any player or team, ALL ten contienents must be taken. Why reset the entire server when we can just reset one continent? Say Vanu is sorrounded, they're basically dead. New Conglomerate has 6 bases, and Terran have the other 4. In order to balance the game, suddenly, a continent with the best NC:Terran unit ratio (6:4 in this case) is 'wiped'. All players on it are ordered to go to the nearest warp gate and evacuate the country - or possibly a dialog pops up that allows them to warp to any of the other continents. Afterwards, the whole continent is given to the near-dead Vanu. While they're weak and have suffered considerably, they're in a position where with decent tactics they'll be able to try and reclaim some more bases on other continents.
This would be a good compromise - the game would still be endless, but there would be a sense of 'victory' by forcing the players to get a free-continent. You could even count how often this happens.
Anyway... yeah, that's my idea. I certainly think that a time limit (whether it be 5 minutes, 5 months, or 5 years) would be a bad idea.
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