When I used to play WOW, I had a few ideas that I never really fleshed out, but that I mused over in dozens of ventrillo conversations while not raiding (or rather while my friends weren't raiding, as a college student, I rarely had much time for it). One of these ideas integrates the concepts of instanced play, guild warfare, crafting, and concepts from the Hyjal instance and Warcraft 3. I also admit it blatantly steals elements from City of Heroes/Villains, but what doesn't WOW steal from?
Let's say you have a guild. At the moment, it's purely a social experiment with some limited item sharing via guild bank. Mostly it's just a pool of like minded individuals who can raid at a similar time, but honestly little else. It's a social group that delves into playing with specific game elements when it is convenient for the group. That is, in and of itself, totally unfulfilling. Guilds should be a presence in any game, and should have meaning, prestige, and honor.
What I propose is that each guild have a base of operations. Guild members are taxed or donate resources to build buildings which in turn provide resources or bonuses for the members. For instance, players pay to build a town hall and train and recruit peasants to work there. The town hall generates gold or increases the amount of gold players earn from mobs or some other tangible benefit (raw gold generation is probably best for this). The players then can build other structures, such as barracks which create NPC mobs to defend the guild base, or temporary npc allies for quests and missions (the latter would be hard to balance). Perhaps even have alchemy/blacksmith/tailor shops that sell rarer goods than the NPC towns or can perform research to generate patterns that are rarer than what is sold by any other NPC vendor. These bases should be a signficiant time and resource investment and should provide some tangible benefit that is worth the investment.
Now, these benefits shouldn't be without cost. Maintanance fees, quests to make things operational or keep things operational and other tasks should be part of this system. Nothing should be built once and maintained permanantly just because it was built. In this way, inactive guilds will have their bases degrade to a more simple level (obviously the most advanced levels should degrade much faster than the simpler ones, and a town hall should be able to support itself in a limited fashion).
Additionally, guilds who have bases are open for guild warfare. There should be a somewhat involved system for "keying" a guild to an enemy base (bases should be instanced) and these bases should have a limited number of attacking and defending players (40 vs 40 could easily work, but it should scale with the size of the base. AV would be a good template in some aspects for this). Once the guild gains access to the enemy instance, they can raid it for a short period of time (say 24 hours) before they have to redeclare war and rekey themselves. Any damage done to the base has to be repaired by the guild whose base was attacked and any buildings destroyed have to be rebuilt. There should be some cost or involved questing required for guilds to declare war, and steps should be taken to avoid abuse of this system.
Many aspects of this idea could be expanded to further involve players. Making buildings crafted out of craftable parts would be interesting. Requiring 400 iron nails, 20 enchanted bricks, 50 tanned heavy leather, 10 rifles, etc to make a barracks would stimulate crafting and the economy in those areas. Guilds would have a resource drain and resource needs beyond just raiding (which is intensive for potions and other buffs but not in much else, besides grinding instances for gear). Buildings should also take real-time to build so that you can't put up a top tier base in a couple hours, it should take weeks of real time to get a base completely built up (and weeks of raiding to completely destroy the base if you are declaring war, however, it should be simpler to reduce it's efficacy and force the enemy to defend it by say killing off all the peons or killing off specific NPC's).
The system would require a careful hand to ensure that it wasn't exploited and to fix potential holes, however, it could be a very fun and involved system that would prevent the need to push out yet another expansion pack.