I think FOSS is uniquely awful for game development. I applaud anyone willing to donate their time but when it comes to games I much prefer pay-once as an attribute to focus on.

Game devs need to eat and donations are unreliable (I used to help run a community funded MUD - we admins often ended up paying server costs out of pocket) so I focus more on finding good games that avoid ad or microtransaction based revenue models so the most reliably good games I find are either shareware/demo before you buy or just a straight up cost.

I definitely appreciate OSS games though and there are good non-free OSS games out there, most people aren't dicks that reshare the source after purchase.

I don't play it that much, but Mindustry is probably worth looking at, a factory-building game in the Factorio mold.

https://mindustrygame.github.io/

Unciv is a reimplementation of Civilization V (with simpler graphics).

https://github.com/yairm210/Unciv

Not in F-Droid, but open-source: I very much like Cataclysm: Dark Days Ahead as a game, but it's really better-suited to a PC, which is the platform it was originally designed for. It's compute-heavy, and while the Android adaptation is usable on a touchscreen, it benefits from having a Bluetooth keyboard or something. Open-world roguelike. You can just download the apks directly. This has a steep learning curve; think Dwarf Fortress. Lots of depth, though!

https://cataclysmdda.org/

I'm not actually all that rabid about the touchscreen UI. Probably better solitaire UIs out there, but PysolFC is a fork of Pysol, a long-running Linux solitaire implementation. It has a ton of different games; my favorite is Eight-Off, a game that's somewhat similar to Freecell, but less-widely played. I think that solitaire games are a good match for a phone platform, so worth having it or another solitaire game around.

https://pysolfc.sourceforge.io/

My own favorite is one that you mentioned, Shattered Pixel Dungeon, which I think is a good adaptation of roguelike gameplay to the touchscreen. Its creator posts on a Threadiverse community here, !pixeldungeon@lemmy.world.

Vector pinball is quite fun, if a bit simple

Mindustry.

I think Anuto TD (a tower defense game) is the best I've come across.

I also really like Gauguin (Sudoku-esque) and Lexica (word game) on the more casual side.

Don't know that it's on F-Droid, but Lichess is also high quality and open source.

Endless Sky

Stone Soup Dungeon

Endless sky is a spaceship sim that I started playing recently. It's well done, and there are a bunch of mods available for more story, ships, etc.

Unciv is a great demake of Civilization 5

the ones i play most often are Forkyz and Simon Tatham's Puzzles.

If you like Shattered Pixel Dungeon, there's also Dungeon Crawl Stone Soup on F-droid.

Space Trader

Open Flood (puzzle game)

AAAAXY is interesting for a weird mindfuck puzzle platformer.

Anuto TD

Frozen Bubble is a solid Puzzle Bobble clone

I've made a small foss shoot 'em up with bevy and plan on porting it to android at some point

Super Metro Mega Wars

Shattered dungeon

TuxCart is pretty good!

Cards With Cats is a solid Hearts game.

PySolFC is a slightly clunky but workable solitaire.

I love Lexica, a word game similar to Boggle.

Finally, CrossWords is a good scrabble clone, though I've had issues with network multiplayer buggering itself. Good for local play still.

Freeblocks Pixel wheels

And retroarch