I reckon you should base guilds partly upon the feudal system - Each player could provide some troops for the general use of the guild (in other words, the "Grand Army", commanded by the king. Of course, the tribute should be used as well. In fact, how about a "guild" resource pool, that people above a certain rank in the guild could use for, well, anything. This would be the tributes that pile up. Anyways, some good ideas circulating in this thread. I wanna see some of them in the game.