OK so the fact that one can't include manage a subtree as its own git repository is indeed not very convenient. But I'm not very enticed by the idea of complexifying project structure with nested entries.json. What do others think?
We haven't really given much thought to reusable engines built *within Superpowers*. I mean, there are project templates (which you can put in systems/game/public/templates) which are one approach, but it's not quite the same. Maybe your engine would be best built as a plugin for Superpowers Game? It could expose its API like the sound play exposes its API for instance: https://github.com/superpowers/superpowers-game/tr...? I feel like that's a good match.
It would be a very simple plugin that only have a typescriptAPI bundle and no editors or data folder.