Just the stuff that I've found neat with some text on why you maybe should try them.
I won't say that this page only lists niche games but it certainly won't list any super-popular ones because you already know about them.
Playtime: ~50h
Playing the first game is recommended but not required
Atelier Sophie 2 is my all-time favorite turn-based RPG period, and here's why:
On the first glance, the combat in Sophie 2 is nothing that ordinary. You have your characters, you try to bonk enemies to death before they do that to you. It's well-balanced and looks good but that's about it. Well, it is about it until you get your 4th party member (you have 3 active ones and 6 total members).
Once your back bench gets introduced you gain access to TP (Technical Points, or Toilet Paper idk) which you gain during combat (with some ways being faster than others, for example hitting enemy weak points). These points can be used for 2 things: support attack and support guard. Support attack allows you to swap your current character with one from the bench and do an action with each of the two characters, effectively giving you 2 turns. Support guard can be used on reaction when an enemy attacks one of your characters to swap the target with a backbench character and automatically puts them in a guard state.
This one simple mechanic might just be the best in all of gaming. It's really simple to understand and use on a basic level, but it has a ton of depth. You should not spend your points as fast as you get them because Support Guard is a really strong defensive option, but at the same time doing Support Guard too often will leave you without much damage. On the other hand, Support Attack can be used defensively too - switching a low-hp character to the backbench insures that no enemy attacks will hit them, but you can still use healing on them via your current active characters.
All-in-all, this simple mechanic creates a great tug-and-pull and makes every non-trivial combat encounter an engaging, delicate balancing act. The game has other great combat mechanics - like auras and time cards, but this section is already too long so we'll move on.
You play as an alchemist so you might guess there's a lot of alchemy in this game. The synthesis system is an open ended-puzzle that ties very well into the overall game - it's practically the only way to get good gear and usable items, which matter much more in combat than actual levels of your characters.
The puzzle itself makes you fit pieces on a square grid. The pieces are the ingredients which you collect during combat and exploration, with a wide variety that can be used for each recipe - collecting and picking right ingredients is as important as fitting the pieces on the grid afterwards. Then the actual process is quite mind-bending and varied. Different recipes have different grids, with some spaces blocked, and different requirements to the amount and way the cells should be filled. It's simple at first glance, but like most things in this game, has a lot of depth and opens up with time as you figure out how to effectively utilize alchemy skills, catalysts and ingredient links.
I do have a nitpick with the system though, and it's the fact that you have 2 alchemists with separate alchemy levels and recipes in this game. While it does make narrative sense why they should be separate, in therms of gameplay it's just annoying - leveling 2 different bars to get the best gear for all characters is just tiresome, and there is 0 mechanical difference between the actual process of synthesis between the two alchemists. If not for this this I wouldn't put the redundant/underdeveloped mechanics tag on this game.
Try this game even if you have the slightest bit of interest in it. Atelier is a weird series with a lot of variation, quality included, and I definitely think that this is the best atelier game, although I have yet to play all of them.
Playtime: ~25h for main story mode. Has challenge and endless modes that are sufficiently different to allow for much greater playtime, especially considering the mod support.
Cavalry girls is the definition of a hidden gem to me. Steam absolutely shafted its marketing by putting the adult-only tag on it, even though the game does not actually feature any heavy violence or sexual content. So people that aren't interested in hentai games look at it and go "oh, another porn game, whatever" and people who expect hentai buy the game, see no sexual content and refund it.
Cavalry girls is, first and foremost, a top-down mecha action game. Your goal is to defend a city for 100 days, so gate defense missions are the most common, but later on in the game you will get access to attack missions where you destroy enemy convoys, bases, and bosses, so it's not all tower defense.
The combat is implemented exceedingly well. You need to manage many variables during it (movement, running charge, crouch/uncrouch, reloading, accuracy, ability and grenade cooldowns, etc) this, combined with pretty good audio design, for a lack of a better description, really makes you feel like a mech pilot.
Different mecha (each of your cavalry girls pilots her own unique mech) also do feel very different. This is due to vastly different modes of movement (skates, legs and tracks really *are* different in practice. It's not just +X movement speed) and weapon equip ability.
The customization is, also, very good. Each mecha has 2 main weapons, 2 melee ones, grenades, leg/side/shoulder/body/core modifications AND on top of that your equipped weapons have their own customization slots. And each upgrade is not just incremental stat bullshit you won't really feel, all of them are impactful in their own ways, and are needed to handle different types of missions (attack/defense) and enemies.
The game has a lot of miscellaneous mechanics between missions (crew affection/stress, enemy deconstruction/crafting, production, tax evasion, etc). Most of them feed into the customization system and add some long-term planning to the game, which is a welcome addition.
All in all, even though it's not perfect, i still highly recommend Cavalry Girls. Especially for its price, the game is a steal, and you get to support a small indie dev team.
The following recommendations are a work-in-progress