Hmm. Of the stats, Willpower, Artifice, and Strength for Non-Dwarves aren't touched. Hmm. Workshop could take part-time workers to raise Artifice.

Willpower I'd suggest a 'meditation grounds', but that's also what the Royal Gardens are for. Could possibly add chances for defensive spells-more so slightly than the Sorcery School. I can't escape the thought of Dryads patrolling it defensively.

Maybe the Castle could have an attack to make it more worth its cost? Or the Veteran Guards upgrade?

I was thinking of the "Ruined Altar" building as a henge for Cultists and maybe also Rangers and Barbarians. Maaaaybeee Healers as well, so if someone goes Agrela/Dauros they can still use that building (and Healers are closer to nature than Monks, as their temples have plants in them).
Adds a small chance that they can summon Daemonwoods, Strangleweed and Dryads while being there.

maybe make use of the sewer and goblin buildings and have them able to train friendly verisons of well the normally hostile counterpart

Bulid monster lairs. That would spawn hostile mobs like ordinary lairs. More simple and determenistic version of otherwolrd portal