Animorphs!

Okay, an adventuring party, but you're all playing druids. Either you're actually druids or, more likely, you're in a super-low-magic setting (so bards are actually rogues with a social focus subtype, no wizards, sorcerers, clerics, etc., and monks have their ki abilities nerfed to shit). Some beast god, artifact, or alien gives you Wild Shape. Go.

This ability takes the place of some other class-defining ability, or else your abilities are already nerfed a little bit (e.g., you're playing in Grim'n'Gritty mode so your hp doesn't matter as much, or the whole campaign caps at level 4, whatever) - but best to manually nerf everyone.

Grim & Gritty hp/critical rules.

BRB has no supernatural abilities, so Path of the Berserker is really the only option. Rage sucks now. Anything that improves criticals is gone.

BRD, CLR, DRD, PAL, SOR, WLK, WIZ are gone.

FTR gets no magical archetypes.

MNK gets no obviously-magical powers or archetypes, ki is just a resource now.

RAN gets no spells or supernatural abilities.

ROG gets a hugely reduced sneak attack and no magical powers.

PCs will be sad, but then also they get Wild Shape as a druid of their level of the Circle of the Moon! The caveat is they have to make some kind of meaningful contact with the animal before adding it to their repertoire and maybe they have a limit on animal forms of level + Wis or something.

For added nonsense have it so a BRB has to eat the thing's heart, a FTR has to best it in combat, MNK has to touch it and meditate for a minute, RAN has to live alongside it for a moment (or in its den, whatever), and ROG has to steal its skin or something.