Hardcore Mecha. Indie Side scrolling mech action game. It's pretty fun and sometimes can get out of the mech and do mission on foot.
Earth's Dawn. Imagine if Vanillaware made a Sci-fi action game. This would be it. Nice hand drawn visuals. Almost Gears of War ish in setting. Also on Switch as Earth Wars.
Abzu. It's single player only Journey and underwater.
Assault Suit Leynos. A great remake of the SEGA Genesis/Mega Drive cult classic. Has some QOL improvements and new hand drawn art.
Chorus/Chorvs. Also on modern consoles.I explained this in the Star Fox thread. Story driven space dog fighting game with some Zelda like dungeons. You're basically female Darth Vader defecting from the Empire.
Curved Space. A fun top isometric twin stick shooter. Sci Fi. Mostly shoot space spiders.
Metal Wolf Chaos XD. It is an HD port of the original Xbox mecha shooter developed by From Software. No this is not like Armored Core. This is just a shooter. You play as Michael Wilson the President of the USA. His vice president has turned evil and turned the country against him. YOu must clear your name by naturally getting in a mech and shoot everything.
Nex Machina. Maybe not hidden as it is from Housmarque but this is the best twin stick I ever played and is inspired by Robotron 2084.
Matterfall. Another Housemarque game but a side scrolling shooter. Pretty good.
Ultra Age. Rough around the edges with some bad VA but a solid action game. In some ways it's a Nier at home but not quite that.
Caladrius Blaze. It's a solid Shmup but the downside is the anime girls lose clothes when you do better in the game. No nudity thankfully but if you can handle that cringe. Solid shooter.
Battle Garegga. Not hidden to fans of the genre but it is to most and a fantastic game, Brutally hard tho.
I could just list shmups tho lol I won't. Too long.