It would be interesting if there were a specie accessible only through glade events. Especially with the new choice system, one of the rewards to some of the fishmen related events could be to invite some fishmen into your colony.
The fishmen would be outside the regular economy and able to perform jobs which the others species cannot. Those jobs would be available in special fishmen buildings that would unlock automatically as soon as you get fishmen villagers.
For example in a Fishmen temple, fishmen workers could prolong the current season through their arcane rituals. In a fishmen foraging camp, the fishmen would venture into their underground networks and bring back random resources scavenged in their caves, providing you with an infinite, if random, source of raw material. Or perhaps they could reduce blightrot or hostility gradually during the whole year.
These useful powers would be counterbalanced by the fact that the fishmen would only be available in limited numbers as glade rewards and would increase impatience and decrease other villagers’ resolution by their very presence.
In order not to have to implement an entire new specie, the fishmen would basically have no resolution, so they would require no housing would not consume complex food or services (the fishmen are used to living in the woods and don’t need any amenities), would not be able to work outside of fishmen buildings (the fishmen refuse to adapt to the settlers’ life) and their death would not anger the crown (quite the contrary).
The fishmen would be a gamble which would give you access to rule bending tools otherwise difficult to get but would also make the crown angry and your other villagers unhappy. It would pave the way for all sorts of map modifiers and pillars which would encourage using them by lowering the penalties associated to them. The idea being that having some fishmen in your colony should give it a special vibe and make it memorable.