I obviously have a few hours today where I have nothing productive I can be doing, and I'm too lazy to clean, so I'm going to spitball ideas that have been rattling around.

I think a C64 game engine that's generic enough to be useful for many kinds of games, which is also performant enough to make good games, and has the features a game engine should have in 2023 on the C64, is doable in a mix of C and asm -- C for high-level control, asm for performance-critical bits.

I also think that most of the asm stuff has already been written, and can be pulled together from various sources. There are lots of music drivers, there's a really cool new sound effect driver, there's that neat trick I experimented with (but did NOT invent, I have never invented much of anything new for C64) using the NMI timer as a second thread of raster IRQs.

Make a good tile engine, build a basic entity-component system, document how to make entity types and set up their behaviour, it's all very doable.

@martin_piper has a good sprite multiplexer that could integrate into such a beast. It could detect NTSC65/NTSC64/PAL for timing of the music and rasters, too.