Try to turn it around and look at it the other way. One possible option would be just the opposite; rather than expanding, the map would shrink. I imagine this option as some kind of diplomatic aggreement.
I will not count with 2 types of maps: "standard" with borders and "civilization" one when one side of map is joined with the one on the opposite side. I presume the standard one, but I dont think this would be a problem in the other one too.
Example in standard: We have 2 vs 2 large super map, each player starts "at corner" and over time develops an empire. So at some point, call it Time of War, 4 empires stop expanding and meet each other in the middle; players can then vote to shrink the playground to the middle section. Once shrinked, you can only issue orders in the new play area, but your previous empire is not lost; trade still applies and you can cast spells...only your actual active playground is smaller.
Propably a cosmetic feature at first glance; but I believe that players, once they have focused view, start to think a little differently. They have smaller territory to work with which is much easier; and every plans will count only with these territories (since they will be the only possible). They will fight with their "starting town" on the focused view like with their capital; the difference is, if they should fail and lose the game, they still have an empire behind them.
Not that I am discarding the "expanding" concept but this one looks interesting too.