ToME WikiRavenRed: A slight change to the newly-reimplemented-in-CVS Symbiosis.
To add a bit of flavour to the molds and jellies theme, when they are "summoned" they are in fact grown on a cadaver (making an assumption that they're all saprobes of course). The "Summon Never-Moving" ability gets re-branded "Seed Spores" as an area-effect spell centred on the caster that "raises" any corpses as blobby servants to the symbiote (in game terms, deletes the corpse and summons a jelly or mold as a servant of the player).
At higher levels of Symbiosis, the spell could also do a relatively small amount of "PURE" damage to creatures in the area of effect. It should never be a first-choice attack spell, but should monsters die from its casting, they should be more or less instantly "raised" as new potential symbiotes. This changes the focus for Symbiants to become almost organic necromancers rather than summoners.
2c worth and counting...
KhymChanur: Interesting idea, I like it. Maybe the corpse could influence the type of never-moving to grow. For example, the corpse of a monster that deals fire damage would be more likely to grow a mold that does fire damage, and less likely to grow a mold that does cold damage.
Also, maybe there could be two variants: slow-growing and instant. Slow-growing would cost less and be available at a lower symbiosis skill level, but you'd have to wait a while for the corpse to turn into a mold. Instant would act like summoning does now. Hmmm, and the slow-growing could immediately turn the corpse into a defenseless "moldy corpse" that takes a while to mature, so you either have to guard it until it matures, or leave it behind and hope that no monster comes along to kill it.
KhymChanur: And a variation on "Seed Spores" would be "Start Culture": you hack some pieces of flesh off of a never-moving's corpse, then plant a piece of it onto another corpse; the corpse then (immediatly or after a delay) turns into a never-moving of the same kind. This has the advantage of getting exactly what you want, but the disadvantage that you 1) need the corpse of a never-moving, and 2) have to use the hunks of flesh before they rot away in your pack.
RavenRed: Whilst I like the idea of making Symbiosis a school in its own right, I'm not sure whether making corpses into virtual summong time-bombs is quite what I'd envisaged. This requires severe inventory and dungeon management (almost like Rogue traps) which changes the class significantly. Not that I'm blanket opposed to the idea.
KhymChanur: And a not so little change, but if we're going with spores, why not change symbiosis from dealing with never-moving monsters to dealing with slimy monsters? It doesn't make much sense to be able to symbiotize with a skull druj (as an example, don't know if they're making it to T3), but not be able to do so with a moving oozes. That would also increase the range of monsters players could use, since there are more slimy monsters that move than non-slimy monsters that never move (silent watcher, drujs, tangleweed, a few others).
RavenRed: I agree absolutely. Playing a melee symbiant is currently extremely limited. Having an Acidic Cytoplasm as a symbiant changes the issue, however... thanks to Matt's new and flexible system, we can be quite fine-grained in what monsters we allow and what benefits they bestow. I also agree that jellies, molds and fungi should basically be the only monsters available. BTW, how do multiple symbiote slots sound?
ReenenLaurie: This is a much more "rounded" idea than the way symbiosis is done at the moment. The whole idea of a symbiot is drastically out of theme though. But it reminds me of the Pirates of the Caribbean 2 guy. Should get serious penalties to char.
DarkGod: Yeah I like your way. And Reenen, well we could have it set charisma to 3 when a symbiote is worn 
PatekShoggot: Perhaps a hit to hp when manually taking off a symbiote? Would prevent a two-step 'take off symbiote, buy stuff in shop, wear symbiote' deal. Explainable via the pain induced by ripping its tendrils out of your flesh... Hrm, and wouldn't removal of a symbiotic creature cause it damage, as well? Quite possibly killing the symbiote... As symbiosis stands, it's not hard to carry multiple symbiotes, swapping between 'em as needed. Killing the symbiote on removal and health hit when removing a symbiote might offset the wearing of multiple symbiotes? Also consider having the maxhealth of each symbiote lessened, for each extra one worn (less resources from the host available to each).