I have several things to state on this topic.
More resources are always a plus, but they need to have varying degrees of usefullness and requirement. Most all towns should have access to a small amount of basic resources, (i.e. wood, leather, metal, food), and it should allow them to create most units. However, it should take longer, and they should not have any bonuses (aside from racial ones of course). Gaining access to magically infused crystals, dense iron deposits, or gold, should give noticable bonuses to the Empire/Kingdom and/or your units (that use items/equipment that can utilize the bonus).
Trade should be required in order to give your Empire/Kingdom access to those resources (i.e. a new town you've built, several turns away by fastest scout, shouldn't be able to utilize any bonus resources unless it has a road to the rest of your land). Likewise, trade between players would matter, since exotic, or rare resources would give large bonuses to units/Empire/Kingdom/costs.
This would obviously entice people to go to war for certain resources, however, taking a town, even if it is roaded to your Empire/Kingdom, should take a certain number of turns to allow the infrastructure to start working for your Empire/Kingdom (say three turns before you gain the bonuses). That way people wouldn't just temporarily take a city or town and try to gain peace by offering the city or town back. It'd be neat to be able to chose sack for a town to destroy the town or hamlet (I don't think you should be able to completely destroy large cities, without a magical destruction), and gain a certain amount of said resource (i.e. your Empire/Kingdom makes 1.5 points of mythril, so your stores are only at 80, you sack a major mining town that you know is too far out from the rest of your Empire/Kingdom, and sack it, gaining 300 mthril).
The system for resources should be based on a certain amount per turn, (i.e. .1 for trace amounts, .3 for deposits, .6 for major deposits, 1.2 for a developed major deposit) and build up a pool (i.e. which could be held in storage buildings, which would also make raiding useful, likewise, attacking trade units could allow you to gain say 1% of a(n) Empire/Kingdom's resources). This would give greater flexibility, and allow you to stockpile for campaigns, while being fairly simple and not too complicated. All cities of a certain size should share resources, but new towns, captured property, and unroaded towns should either only have what they create (and thus not share it with the rest of the Empire/Kingdom), or at least have greatly reduced rates and building power.