One guild at a time? HATE the idea. Despise it. Loath it. Its repugnant to me. Might as well transport me back to 1980 and force me to play a game like Gauntlet where I can only be either a Fighter, Mage, or Ranger. I'm also not a fan of arbitrary rest periods between quests.
That being said, I think the TES franchise could do with an overhaul of the Guild mechanic. If it were my game to design, I'd give Guild interaction a front row seat up there beside "main quest" in terms of primary reasons to play. Here's how I'd design it...
- Increase the number of guild/factions, but pick carefully which ones the character can "join" versus build esteem with. Further faction examples include Witch Covens (a la Daggerfall), Twin Lamps, setting specific religions, setting specific political factions, Beggar's Brotherhood, Imperial Civil Service, Merchants & Bankers, Bandits and Highwaymen, etc.
- Those guilds you can't "join" we'll call "factions". You can do quests for them, and get rewards, but you only gain "esteem", instead of rank.
- Build a matrix defining guild relationships with each other. Some are polar opposites (ie. Merchants vs. Bandits), where as some are only suspicious (fighters guild vs mages). Others are purely neutral toward each other (Mages and Imperial Civil Service). If you gain in esteem with one faction, you gain suspicion in enemy/suspicious guilds.
- Build enough suspicion in a guild/faction and you'll be kicked out, fined heavily, or be barred from further quests or guild services.
- Design quests to mitigate suspicion thus providing lore satisfying reasons to be in uncooperative guilds. Example: "bribe, coerce, or convince a local bureaucrat to legalize gambling halls... completion of which reduces suspicion and raises faction standing in both the Merchants Faction and the Thieves Guild since they'll share the new revenue stream".
- Introduce moral complexity by interaction with various factions. Example: Fighter's guild wants you to escort a local merchant to the next city - complete quest for raise in Fighter and Merchant faction ratings. Later, the fighter's guild offers a job to shake down a local merchant who's behind on gambling debts - complete for raise in fighter guild and thieves guild standing while losing esteem with the merchants.
- Create several repeatable quests per faction to keep the esteem/suspicion interplay alive (eg. delivery, collection, escort, convice/coerce/bribe, cave-clear, assassinate).
- Create multiple unique rewards per faction/guild including "perks", permanent buffs, skill increases, or unique items. Examples: "Dibs" (Beggar's Brotherhood) - player may rest in cities when out-of-sight of guards. "Imperial Imperative" (Imperial Civil Service) - player may borrow a 15% faster horse from the postmaster's office. "Blessing of the 9" (Imperial Cult) - player may use a Divine Intervention once a week that does not depend on Mysticism skill.