The possibility of hiring NPC mercenaries sounds great, and I hope it is included in the game. In my opinion, there should be several mercenary organisation that are willing to help the guilds in exchange for money. Some of these would be composed my elite warriors and experienced fighters (and thus, their services would be really expensive), while others would be cheaper and more accesible for guilds which do not have plenty of resources but are in need of manpower.
Also, hiring another guild to help during a war could be nice. The terms of the contract should be agreed by the guild leaders (the price for hiring the guild, the way the gained items are distributed, etc). I think this could have a great impact on alliances and the way a guild sees the other, and could be also very profitable for those guilds who are willing to fight for money.