I mean first of all there is all the other
Tsukinomizu Project games.
Though the "Naedoko Dungeon Chronicle" and "Naedoko Demon's Ground" ones might not fit the harem part(admittely I wouldn't consider Into the Nightmare harem in the first place.), they are more pokemonish (except you birth your monsters after mainly loosing and getting raped by tentacles), though in terms of combat mechanics they still works in similar way in the end. Forget-Me-Not Gemini Fort, though barely have any combat at all though(as in maybe 2-3 fights in the entire game), that one is more of a mystery solving game, still featuring 4 heroines as the player party though, so in that sense as much "harem" as Into the Nightmare, but certainly the most different one of their games
The rest of them are fairly similar to "Into the Nightmare" though I would say. All their games take places in same universe, with the chronological order being the same as the release order of the original JP versions. Still all completely separate stories, so dont have to play them all or in a specific order, to get what is going on, though you might notice some characters coming back, or other interesting lore stuff fitting together if played in order.
