Smaller alliances = keep mercenaries!
In a game that is supposed to be all about constant war, giant alliances of 5,000 - 10,000 is ridiculous.
Instead of new guilds and alliances trying to fight and gain ground, they just go with "if you can't beat them, join them" mentality.
Make the big alliances really have to pick between guilds, create some tension, and start some fights!
If a guild isn't contributing to the alliance there will actually be incentive for the alliance to kick them!
It could make the territory control more like the game 'Risk'.
Sure, a few alliances could agree not to attack each other.
But someone could always betray and attack anyways. Multiple alliances could even coordinate group attacks/ambushes.
Even just the possibility helps create more tension. Someone might be worried about getting attacked, and decide to launch a preemptive attack instead!