Also even if it is a ground up rewrite, the look and feel still matters.
Try creating a 1:1 dupe of a Hermes bag or a Rolex and see how their legal team reacts (even if you call it an OpenBirk)
- OpenArena
- Chip's Challange and custom levels pack
- Freedoom+Blasmepher for Doom/Heretic
- LibreQuake
- Supertux2
- Oolite
- Kgoldminner/XScavenger with level sets
- Frozen Bubble
- Any X11/console/9front sokoban clone. Everyone reuses the same level set over and over.
Clean room reimplementations of software projects have been tested in court and are legally fine