A lot of old games have become unplayable on modern hardware and operating systems. I wrote an article about how making games open source will keep them playable far into the future.
I also discuss how making games open source could be beneficial to developers and companies.
Feedback and constructive criticism are most welcome, and in keeping with the open source spirit, I will give you credit if I make any edits based on your feedback.
You could do open source engine, proprietary assets. Like the original doom 1, 2, and 3. You can get the engine for free, but have to pay for the art assets.
https://en.m.wikipedia.org/wiki/List_of_open-source_video_games#Open-source_games_with_non-free_data