Similar to Roller-coaster Tycoon etc.

The only fully open source formerly commercial game I am aware of is Warzone 2100. And for a while I think they didn't have access to music or video files.

also https://colobot.info

OpenTTD lets you use either the base graphics, which requires Transport Tycoon Deluxe, or OpenGFX graphics, which does not.

Star Control II / "The Ur-Quan Masters" might count as well.

TUQM is not actually a reimplementation, the source code and assets were legally made available by the original developers as they retained the rights. What they didn’t own was the name “Star Control”, hence the project using a different name.

Probably my favourite game ever, we owned the original with the map and manual.

I replayed it a couple of times as the TUQM, still loved it.

I tried replaying the original X-Com, but it had serious quality of life issues (squad members not remembering gear assignments between missions, no keyboard shortcuts during missions) so was just too tedious for me.

Apparently there is a reimplementation of the game out there which fixes these things, but at this point in my life my time is too limited.

> I tried replaying the original X-Com, but it had serious quality of life issues

There's Xenonauts, if you want X-Com, but modern.

https://store.steampowered.com/app/223830/Xenonauts/

> squad members not remembering gear assignments between missions, no keyboard shortcuts during missions

I still have a notebook that contains squad member names and loadouts. I used that to quickly equip soldiers for missions.

You can tell the changes by the crossed out text. When I got plasma rifles. When I got blaster bombs. When my main assault guy caught a heavy plasma hit in the face.

Good times, but I’m glad they remember the load out in the new games.

One Hour One Life (http://onehouronelife.com) is public domain (software and assets). It's not playable without a healthy multiplayer community. The developer sells access to the official server.

what about quake3? Does it need assets belonging to id software?

Here is another one:

https://play0ad.com/

OpenTyrian is ported from the original Tyrian code and the original assets have been made freely available, the original graphics artist even made a remastered version of the tileset [1]

[1] https://lostgarden.home.blog/2007/04/05/free-game-graphics-t...

Same with a friend‘s re-implementation of Battle Isle 2 (https://www.kitana.org/arena/bi2/bi2.html)

You would need the original or the GOG version.

oh wow haven't seen this in a long time. I played Battle Isle 2 like crazy back in the day. None of the sequels manage to get me hooked though.