Although a relatively new ability introduced in Gen VI, Symbiosis has come up several times across your blog history (see https://pokemaniacal.com/?s=Symbiosis) and it sticks out as an ability that is oddly specific, to the point it’s arguably not useful except for passing on items to allies in double, triple, etc battles that have consumed their own consumable held-item. How would you rethink this ability or create a new ability that captures the concept of symbiosis (let’s just think about mutualism or the purposes of simplification)? My immediate inclination is that if a Pokémon with Symbiosis has a held item, it also copies the effects of that item (but not the item itself) onto an ally once said item is consumed. Extra credit: this ability could be retconned as a Hidden Ability for Slowbro and the Bulbasaur evolutionary line.
I think most of those search results are actually related to symbiosis, the real biological concept, not Symbiosis, the Pokémon ability (as far as I can tell, the ability has come up exactly twice, in my reviews of the two Pokémon that get it). That in itself is meaningful though, because symbiosis – biological cooperation, unrelated species living together and supporting each other – is kind of what Pokémon is about. That being the case, it makes sense to want the ability Symbiosis to be something interesting and powerful. As it is, well, it’s certainly interesting; you can pull all kinds of weird, creative tricks with it. While it’s not really a strong ability right now, I would worry that buffing it would have… unpredictable effects (we can’t just evaluate what the Symbiosis Pokémon themselves would do with a buffed version of the ability; it’s all the possible partners and item combinations that might threaten to break something). Something that lets you stack the effects of multiple items on a single Pokémon… if it were actually efficient and had other bonuses on top of that, I think might become oppressive. There is definitely an argument that Oranguru and Florges get kinda screwed over by the fact that not only Symbiosis but all of their abilities are useless in singles (and I am very much including Inner Focus in that statement). At the same time, though, the concept of an ability called “Symbiosis” sort of seems like it should require a partner. I think the better way to help out Oranguru and Florges is by giving them different abilities that do work in singles (Florges only has two at the moment, so we can tack on a third, and for Oranguru… well, to be honest we could just buff Inner Focus; every other Pokémon that gets it has better choices anyway except for Gallade).
If I were designing from scratch an ability called Symbiosis, or meant to express the concept of symbiotic relationships, what would I do…? I suppose it depends on whether or not we care about making it something to do with items. If I had no preconceptions about what the ability was supposed to be, I would have it be something that shared beneficial effects somehow. For instance, whenever the Pokémon heals another Pokémon (including curing status conditions, meaning you can do it in singles with Aromatherapy), it heals itself by a certain amount too, maybe 1/3 of its maximum HP. That seems like it could be useful. If we do want an ability related to items… hmm. Something like what you’re suggesting, where a consumable item has its effects shared with the partner, seems fitting, and I think the power level is only a little higher than where Symbiosis is now.