I thought of something just now that may help a bit - let's call it the "Wipe Progression System". I'm not going to type much so that people's ideas can come out and flourish / propel further ideas.
Wipe Progression
When the player base usually begins to die off after a fresh wipe (5-8 weeks max?), further improvements outside of the rank system become available.
These can begin slowly such as bonus +stats from quests (keep in mind quests must be VERY easy to complete at the beginning as the player base is low - aka "kill Trachyte for permanent +1constitution", "kill Frostbyte for +1 permanent damage", "kill Sleet for permanent steelskin", "clear ToT, DKO, and FT for +100hp", "defeat spire, storms, stone bluff, mithas for ams", etc.) Yes these are powerful bonuses, but keep in mind this is when the number of players is dwindling and the people still playing become friendly with each other for the most part (this DOES happen, right?).
These bonuses can be limited by week/month whatever. The first month of low playerbase opens normal +stats, +hitroll, saves, damage (0/4 of each?) and 100hp, the following months can open very strong abilities such as a rogue being able to cast heal and healing cloud, a barbarian with the ability to cast 2x shield spells, cleric opens +slots, warrior autohaste, etc.
This not only increases the ability for players to play on fewer players, but decreases the value (does not nullify) of equipment, allowing players to explore in smaller groups.
6-8 months into the wipe everyone playing will be 2000hp, 5x obliterating, stun/blind area specials, auto steelskin, haste, nobash, be able to heal, etc. Sound stupid? It is, but there are only 5 people playing at this point so who the bapt cares.