Couple of ideas...
1. How about awarding points for either trading goods with other players or for polishing/motivating? This way, both fighters and non-fighters could gain points towards getting those land expansions. So if you attack a lot, you gain battle points. If you trade a lot, you get merchant points. They both could be used to unlock the land expansions. Perhaps these points would work at a ratio 20-1, or even higher, since maintain an army costs more. But at least this way both sides would be satisfied, and both would actually have something to do after finishing all the quests and conquering the whole map.
2. Or, give battle points to those who actually win the combat. So if the attacker wins, he gets victory points. If the defender wins, he gets victory points. After all, "victory" points should go to the actual victor (including victorious defender), not just anyone who attacks.