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.

Mindustry.

Vector pinball is quite fun, if a bit simple

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.

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.

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.

Endless Sky

Stone Soup Dungeon

Space Trader

Open Flood (puzzle game)

AAAAXY is interesting for a weird mindfuck puzzle platformer.

Anuto TD

Super Metro Mega Wars

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

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.

Shattered dungeon

Freeblocks Pixel wheels

And retroarch