ToME WikiLordEstraven: I find it a bit odd that symbiotes basically have a slot to themselves, and I think I know a good way to change that: different types of symbiotes should occupy different item slots, and perhaps have the same kinds of traits as the items they replace. For example, a deathmold or quylthulg would go in the armor slot, and protect you from damage at its own expense - it would have a certain AC value of its own, and after the damage subtracted from other items' AC, the damage blocked by its AC would be subtracted from its hitpoint pool. That needs a bit more work though, since the symbiote's AC would have to provide some protection to it as well.
Druj, on the other hand, would be weilded as weapons. The eye druj would be pretty useless in melee, the hand druj would be a 1d3 slicing weapon (think claws) plus enchantments, the skull druj would be a 1d6 bashing weapon plus (somewhat heavier on average) enchantments, and perhaps with cutting ability (it has teeth!). Or maybe druj should be wielded as ranged weapons so as to give the eye druj some additional use as well, and could be thrown to inflict damage as well as used to cast spells?
Some symbiotes don't fit neatly in a particular slot though, so I think it might be better to have a number of given slots they could be wielded to. Druj might work as ranged weapons, primary weapons, ammo, or a light source, for instance, and molds could be armor, gloves, boots, helmets... You get the idea. Large and powerful symbiots such as quylthulgs might have to be handled with multiple slots - an emperor Q is huge, for example, and so would take up all your armor slots. (That's the price to pay for being able to mass summon powerful monsters!) A deathmold is big but not nearly so big as a qyulthulg, so it would be body armor or gauntlets and a weapon - as armor it would act like an aura and fight back at opponents (and perhaps attack things of its own accord occationally), as gloves/weapon it would attack along with you and enhance your fighting ability but not provide as much protection.
I think the latter idea (symbiotes wieldable to multiple slots) would be the better and more "realistic" one, however the idea of one symbiote occupying multiple slots at once looks to me as though it could be hard to code, though I really have no idea.
Also, in view of this new stuff, I wonder if it wouldn't be good to allow for multiple symbiotes? This could make symbiosis very powerful on its own, but at the expense of a large number of item slots. Might still be rather unbalancing though, because of all the extra HP, although that could be mitigated by making very large symbiotes incompatible with each other, because you have only so much area for them to occupy. Of course, possessor/symbiotes would pose problems here because of body size variability. Maybe it would be better to just stick with single symbiotes and ditch the idea of a symbiote occupying multiple slots at once.
(BTW, I really need a vote on this... Supposing that a druj could be wielded as a ranged weapon and thrown, should it return to the thrower or explode violently?)
JohnGilmore: I think that this idea has merit - get rid of the symbiotes only slot, and instead wield symbiots in place of regular equipment.
Symbiots have advantages other than resists etc., so they should be quite weak in the resists etc. department.
I think that they should be exactly like regular equipment - one slot, one possible slot. If you want something different, get a different monster! The number of possible slots should also be minimal - there probably won't be any "bow" type symbiots, for instance. Likewise gloves. Armor, weapon, light(maybe), cloak, necklace (hand druj would go here, I think..), maybe a shield. But no gloves, no boots, no rings. Unless, I suppose, there's already a particularly thematically appropriate one. But I think that players should face some real trade-offs as to what to wield - that fantastic random artifact armor with two immunities, or that Master Quythliug with its summon greater demons, +50 ac, and +500(effective)hp? Hrm... I think I'll play like a summoner for a while, and leave immunity to acid and cold out for now... Ya know?
As for exploding - does the druj regularly explode? (like a snaga sapper?) No? Then I don't think it should explode. As for returning, I don't think it should do that either. (unless it normally flies or something) You'd make them just like regular arrows, including stacking (which they don't do right now...) They're advantages would be mighty damage or special effects like fear, poison, cold, etc., thematically in line with their spells/attacks/etc..
One particularly cool one would be reproducing non-movers, which have the special effect of never running out of ammo. 
And they'd be light to carry besides! (you only need one!) Blood sprout sling ammo, anyone? You could even give the ability to cast a spell, and select which of the symbiote's spells would be cast when fired. Have it apply to the whole stack, with a timeout of around 30 turns, after which they "forget" and start casting random spells - or (increasingly more likely) no spell.
LordEstraven: Well, for rings there are always mimics. :P BTW, are you sure Bloodsprouts don't move?
Come to think of it, the ammo slot would best be left out. If it's a symbiote, it has to be at least touching you... And cloak wouldn't do either I think. Amulet doesn't sound quite right, however if you can't find a slot you could always use the excuse of magical incompatibility to keep one occupied... I think weapon is better for druj though.
(Huh, I just had a thought... If a symbiote has to be in direct contact with you, and you were wielding a druj as a weapon, wouldn't that make gloves a problem?)
Oh right, one more thing - maybe damage that would normally be blocked by a monster's innate resists should not be subtracted from the symbiote's hitpoint pool? I think that would make more sense than just ignoring resists or giving the resists to the player. Might be implemented already though, I should know but I don't.