Actually that might come sooner than planned. For the next update I'm going to be moving away from events happening just when you travel with a dice roll, and more based off time and what youre doing.
Though only just learnt what a detrivore is, so going to have to add it to the list.
If theres any other encounters like that you would like to see, let me know, theres probably a whole lot I just dont know or havent thought about.
Could be damn near anything since MiA had such a wide variety of creatures and potential mutations, but since we're still dealing with the uppermost layers I doubt there'd be anything too extreme yet. What I had in mind was an opportunistic scavenger, but as the anime and manga showed even the areas closest to town can be deadly for the unprepared or careless. The Abyss's curse didn't seem to affect creatures as much, so I suppose that could be an excuse for some rare encounters from predators from lower floors, like what happened with the parasites the Umbra Hands were incinerating on the fourth layer, or the splitjaw that chased Riko across various layers. Maybe design a creature as a rare boss encounter like the splitjaw?
Down the lines I really want to look at how populations will shift. Soon you'll be able to clear out packs of beast, say all the wolves. And for a couple days youll have free range to do what ever you want as it repopulates.
Something with that system though I think would be neat, would be migration. Like a single rogue the splitjaw comes up because it turns out theres no more predators up there. This could like you said considered a boss, have its own quests tied to it, etc.
Would certainly add a more dynamic flair to exploration in general. Do you deal with more frequent but less deadly annoyances like the wolf packs, or do you eliminate them and then have to deal with whatever scavengers come to pick them clean, or any opportunistic migratory beasties from lower levels like you said. As far as tying quests to them, I think that'd work pretty well for unique encounters, a particularly intelligent or belligerent example of whatever critter's species wants to snack on your entrails, with weaker versions as radiant quest enemies or minibosses after the first big hunt. That or maybe it could be the opportunity for a Moby Dick styled encounter, where said critter permanently injures you or causes some sort of catastrophe that has you or the Association swear vengeance against it.
That is the main reason I wanted to move away from randomly generated characters. Their will to some point as you kill them off, and I need to boost up the population, Human as well. But any you encounter you could meet again, meet again. The one which takes your virginity, or like you said, you lose a limb or eye to them. For that one day where you meet them again, this time much stronger... Or at lease you hope so, they take the other arm and its time to throw in the towel. Well kick it in.