Alliancehall / Fortress
First, I think you guys should implement a fairly good amount of "guildhalls" or "towns" on the general map. Alliances should be able to take over these towns and make it their own fortress.
Second, players should be able to move their main base with a 12 hour moving penalty. This way every alliance member could station their
base around the fortress with a maximum of 50 members around the base giving some kind of bonus for doing so.
Third, alliances should be able to attack a fortress giving a 24 hour notice period before the attack begins, and if the fortress is lost then the alliance loses the fortress, and all members have to move at least 1000 blocks away. Every alliance member within 100 blocks of the fortress has their army automatically tied to the fortress and get to defend their fortress while all opposing members who want to attack it can attack it.
Fourth, it takes a large amount of resources to maintain a fortress. If all the alliance members combined cannot meet the weekly or daily requirements, they lose their fortress and have to move out.
Fifth, players should also be able to generate an alliance battle map which shows all fortresses and which alliances control them.
In other words, implement siegeing!