I saw Unicorns as a fairly straightforward solution because they are clearly not as favored on their tier as Archers are on theirs, Druids on theirs, and Ents on theirs, for example. Although, perhaps Nargott's idea is better. However, if Archers and Dancers are nerfed, the buff for Fairies needs to be quite significant. Perhaps strengthening the magic of Dryads (giving them a higher skill level) and a special ability for Nymphs? I think that would be good in its own way.

A +20% increase for Fairies is a fairly significant buff (against the backdrop of a -10% grade nerf for Dancers and a -25% grade nerf for Elves). Because it also affects the un-upgraded units, which is important at the start of the game. The basic Wasps deal 20% more damage on the scale, which will undoubtedly significantly increase their effectiveness when using a few stacks of Fairies at the beginning of the game. I don't think it makes sense to strengthen Symbiosis; it's already good.
However, Wasps shouldn't be overestimated either – yes, each stack will deal 40% knockback (in total for 2 casts), but the damage of the best upgrades of tier 2-3 is significantly reduced, and continuing to use basic Fairies in the middle of the game means giving up on, say, basic Druids, which are no less effective.