I think Turbine, and the community in general, is making this far more complicated than it needs to be. A successful guild system is one without formulas, decay or other such nonsense. The solution needs to be simple, adaptable and fair for everyone.
I propose radical changes to the system.
REMOVAL OF GUILD RENOWN
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1. GET RID OF GUILD RENOWN and its accompanying renown 'score' altogether. This errant concept has been at the root of every problem with guilds in DDO since their inception. All problems stem from it.. decay, guild size, complaints on decay and guild size.. on and on. Get rid of the cause of a problem, you get rid of the problem! So simple. Renown and reward trophies are both immediately removed from the game.
2. The player's total patron favor score now serves as the determinant, and upon member recruitment is immediately added to an INVISIBLE score calculated secretly for each guild; no guild knows what their score is. (this is done purposefully, since guild reputation can never truly be measured by a numerical score) In addition, each member contributes +1 point to their guild, per day, for longevity as long as they remain in the guild. If a player leaves a guild any reason, ALL favor they contributed is immediately removed from the unknown guild score.
*A note on TR's: This is an issue, however just because a player TR's or is 'reborn' gives no additional benefit.. patron and thus guild renown restarts at 0 for that player; however, longevity still applies, and that player gets to keep any +1 per day longevity renown they had when they TR.
CHANGES TO GUILD BENEFITS
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Generally stay as they are, but with changes:
3a. Buff stations now require a ML to use. Meaning you can no longer buff to 400% uber-power right as you step off the ship from Korthos. Low levels generally will be matched better with smaller guilds, and vice-versa.
3b. NPC's generally treat guilds with higher reputation, better. Slight discounts, a few new 'under the counter' items to buy, and additional services are available, on a sliding scale, to guilds with high reputation. Guilds with very high reputations may receive letters from various patrons for special one-time and periodic quests, with special rewards. This is important to give organized guilds something tangible to strive for, in lieu of a pointless 'guild renown' score that serves no purpose.
Obviously these ideas will throw the game and guilds into a brief period of chaos as they attempt to reorganize themselves, but overall should fix most if not all problems with guilds.
I may have more to add to this at some point.