Might be a solution to have more than 2 groups ?
If you implement some restrictions on how the teams (3 players per team would be easiest) should be buildt up, you could get some more strategy into it ? (Diplomacy)
Leave the whole thing running for a month or two, and the team controlling most land is declared winner and award them a prize or something ? (Doesn't have to be so big, but maybe create a hall of honors where 3 top groups are placed each round)