One that comes to mind is to simple tie the death thread system to the classes hit die type. Thus a slinger would have but four threads, while the current discriminated class would have the extra threads of their class (this of course would have to be done retroactively to make a difference to the world, (can you hear the slingers whining fighters, Good they now know how we feel)
As someone mentioned above, a mage and a fighter or some combination of mages and fighters really don't make much of a party because a mage really can't protect a fighter from the nastier creatures out there. When the monsters start throwing out harm and meteor swarm and various other high end spells, you either have a cleric to cast Spell Resistance, or the fighters die. If you look at the spells available to mages, they can give a lot of nice buffs and protections against mid level monsters. They can protect themselves extremely well with shadow shield and spell mantle. They can outright kill a lot of monsters with Wail and Weird. But they can't really do much to protect the fighters.The cleric on the other hand can protect and heal and can, through domain spells, pick up a few of the buffs that are normally only available to mages. Thus it seems like the cleric really is at the center of forming any party headed east. It may be frustrating for the fighter that the mages aren't running around with him, but it is the cleric that is the key to him forming a party. If as part of the spell balancing Spell Resistance was added to the mages spell list as some of the offensive spells were toned down, I think the mages would be much more useful in small parties of 3 or 4.I think most mages would agree that Weird and Wail wouldn't be so attractive if there were other alternatives to cause meaningful damage in a high level fight. This really gets back to the overall spell balancing that Ozy talked about and I agree that it is a major project. I wish the team good luck in this project and I'm happy to wait for it.
Server rules aside, I have always and will continue to group with people despite their overall build and level. It's offensive to me to call such characters "useless", no character is useless. With a little training and skill in tactics learned, even a 2nd lvl commoner can be useful in battle, running around dropping healing potions on the party. The trick is to make certain that every character has a function in the scheme... sitting back and leeching of course is just poor form.