A 5th tier with specialized skills/spells would be great, as an alternative for GMing, but I would think that after some time, the mud becomes geared to GMs, or certain specific classes (that specialize in combat).
This is because at higher levels, more and more important mobs become immune to backstab, poison, directionals, bash, steal, and all the fun skills the individual classes get, and a lot of doors are unpickable.
These mobs are important, they are quest mobs or give tons of xp or gold or eq, hence these restrictions, and usually unpickable doors are because they hold the secret to a quest.
But then again, don't players play these classes for the skills they get? Eg, let's say that the big bad Lion of Harte stole the armbone of Robert, and there's a quest to get it back.
As a thief, maybe I could steal the armbone, and return it to the people.
As a ranger, I might poison the lion.
As a caster class, I might use directionals on him.
As an esper/mage, I could cast negative status effect spells on him.
As a warrior, I might occasionally bash and behead him.
As a hunter, I might shoot him with my bow from a distance, before coming in and killing him, relying on cure critic.
As a priest, I'd buff up and kill him with spells, healing on the way.
As a gypsy, I'd tinker and/or manipulate weapons, then kill him.
But chances are, the lion would be flagged !steal, !poison, !bash, sentinel, and probably behind an unpickable door, the same as many quest mobs or high level mobs. So only pure combat and direct combat skills would work on it, and hence classes that rely on skills/spells apart from that are not as effective when un-GMed.
I guess the question is that would the specialist skills the tier 5 players get for individual classes work on all mobs, or just for the lower level, unimportant mobs?
As a mud focused on quests (that's why I like 4D:)), sometimes some mobs have to be just the way they are, and some doors unpickable. But somehow I feel that there are way too many mobs that are immune to fun skills, and doors that are unpickable, just because they can be set as such, not because they hold the secret to quests and stuff.
Then again, quests like VE, Alpha Centauri, Isis and Osiris, Carthage Bay, work well, and they don't rely on high level mobs or hide behind unpickable doors.
The reason for me GMing is for the T1 and T2 buffs, as well as cure critic, as it makes toe-to-toe combat a lot easier, as compared to a non-GMed thief.
I'd RP a thief, but preferably a GM-ed one