I do plan on adding more content to endgames since that's probably what most people will want to experience! I like the idea of having a mini-dungeon of the pred's insides. That is most likely what I'll do. Good point on willing content. I do plan on expanding the use of companions, possibly using them to feed enemies as a way to escape, feeding yourself to them, or eating them for health regen. The shop is the only friendly NPC that just exists in the rooms, but I do plan on that not being the case.
Regarding haunts, my goal is to have omens increase the chances that you'll encounter monsters, which will basically act as the haunt. I like the idea of the monsters being able to chase the player, with events and other interactions essentially slowing you down. A higher speed stat will make monsters move toward you at a slower rate. I could make certain monsters only spawn with a certain number of omens, giving me a way to add more difficulty and reducing the chances of someone starting a game and just getting one-shot by something. Because the dice are zero-based, it's always possible to be obliterated at any point. Companions could help with that, functioning as saving throws.
Different monsters could have different abilities, like a random chance to teleport, add some global debuff, rearrange rooms, or spawn mini-enemies for you to fight. I can imagine a friendly monster, maybe one that likes to play games, but if you lose, they eat you. Maybe have ones that give you time constraints: Find x item in x amount of passages or I'll come to find you and eat you.
Currently, fighting allows you to attack (might), run (speed), or give up. I think it'd be fun to have an action for each stat, something like:
Attack (might roll): this would also apply to defending. Some monsters fight with sanity.
Run (speed roll): if you run away, they still stay on the map and can chase you.
Flirt (knowledge roll): leads to them being friendly to you, allowing you to feed them, eat them, or even have sex perhaps? Would only apply to sentient monsters, although it would be funny for a player to try to flirt with a plant monster or something like that.
Beg (sanity roll): Less confident on this one... basically, begging for them not to eat you. could add to the unwilling experience. If you succeed then they become friendly. Obviously wouldn't work for monsters like Wolfs.
Another change I want to make is have restarting be less of losing progress. Most likely will be as if you are a new person, allowing you to pick up your previous self's stuff. Gives opportunities for monsters to have more than one meal, which I think could be really fun.