Not some sort of system, but a system that both supports variable levels of competition and is also compatible with the controlled in-game economy.

It's easy to game the system in an anti-competitive manner that way, the present system of playing by the hour isn't fun for a lot of people, and a system of partaking in a finite-time (although non-deterministic time) battle is funner than hours of controlling vs not controlling. What I've proposed provides two variables: countdown time (the higher it is the easier it is to mount an offensive and the harder it is to defend) and flag HP (the higher it is the easier it is to defend and the harder it is to mount an offensive), that can be altered to increase/decrease competition as needed. Lots of players contesting a guild fort? Decrease competition. Not enough players contesting a guild fort? Increase competition. Competition can be decreased by raising the flag HP and lowering the countdown timer. Competition can be increased by lowering the flag HP and increasing the countdown timer. Defense can be helped with higher flag HP and a lower countdown time. Offense can be helped with an higher countdown time and lower flag HP. Finally, the rewards can be distributed at a rate consistent with the in-game economy as a result of the control of these two variables.

How guilds are rewarded and how the reward is distributed within a guild is another question. One idea for this I've had is that cumulative guild rewards can be put up toward, among other items, a public overworld-or-underworld customizable (@Portha: $$$) semi-restricted-access guild territory with one guildhouse room assigned per guild member and the rest a common area. The guild territory could then be contested by other guilds that have earned enough in rewards to put up a challenge of any guild with an exclusive territory. I.e. the exclusive guild territory is also a fort. If the contesting guild wins, they are allowed to toss any and all the other guild's furniture aesthetic and replace it with their own (@Portha: $$$), or they could just decide they really like it otherwise and just rearrange the furniture.

Along with that you could re-purpose historic guild sanctuaries to the overworld or underworld, like MoD, US, Ventrue, Brotherhood, LOA and maybe even JesusFreaks, and make access to certain restricted areas in those territories a prize that comes by winning enough rounds with any particular guild there.

A layered experience I guess I would say. But first and foremost is competition at the guild towers. Then the reward system. And then a tie-in between the rewards system and tiered competition I think would be a big plus. That's a big vision though, and competition at the baseline of guild fort experience is really the most important aspect.

Edit: Oh, what would be cool in addition to that is to, in a library in the guild house, keep a historical catalog of guild info for any guild that ever challenged and guilds that were in possession of the guild territory. Also, the guild territory should have the advantage of being able to order (@Portha $$$) any piece of furniture ever used in their guild house in any room via the Babord Home & Living Personal Storage Facilities, even if it's not available to the general public anymore (@Portha $$$).

Oh Edit PS: Make a post office where players at large can receive mail. Heck, offer post office boxes for a fee and require every Graalian have a mailing address or get taxed extra. Create a mail order catalog called the Babord Home & Living Mail Order Catalog. This catalog may be released semi-annually or by another period. You can make part of the Babord economy involve players delivering anything ordered from the Babord Home & Living Mail Order Catalog, carrying things to a delivery vehicle (or boat, or train, or pigeon, or final destination, etc) for customers at the Babord Home & Living Factory Outlet. Heck, you could even get the Babord jobs to give pay raises based on how efficient and productive each employee is.

Edit 2: And among the delivery tasks: deliver new benches or refurnish the dance floor at Azrael's. Monthly spar champ at Supernick's gets a say in the floor and furniture there.