OpenSourceGames

Endless Sky is an open source space merchant game with space battles, a huge amount of ships and variety, dozens of alien species, multiple storylines, and so much more. I think my favorite part is how accessible it is to contribute to the game; most things like storylines, quests, and ship outfits can be added or modified with a simple text file. Saves are also plain text files, and can be edited, exported, and imported. There are also tools for creating sprites and the game actively encourages mods, which they call plugins. It's such a deep and complex game already, and only one storyline is actually complete. It's available on steam, and a mobile port for Android exists on f-droid