Tax...tax....
Well, the part of taxing someone for ownig a house is fine, although, the feeing part SHOULDN\'T be dev responsabiliti, the player could like, go to a goverment house (npc...) and pay there, he could also pay in advance, so if the person goes on vacation, he has it already paid. Or there could be a agreement on the delay on the payment of two months, for example, after that the house is removed from the player, or he pays a increased tax to compensate the two months (a lot) so this way, in two months the house would be clear for another buyer, or would be payed and in use for the original buyer. Dunno, just ideas....
Also, the army idea is quite nice, after all, the cities DO depend on the taxes. The bigger the traffic, the lower the taxes and the biggest the armies. Even because, cities could wage war against cities, that would be nice hehe >=)