When I was still doing a bare metal thing (before I switched to running atop a Linux kernel) I had things set up so that the extra cores were essentially pretending to be separate "chips":
Core 2 dealt with async blitting/composition and some other misc graphics work
Core 3 dealt with audio mixing/etc
Core 4 dealt with other misc hardware stuff, like interrupts from the USB controller
This was more for fun than utility though. Aesthetically similar to custom chips but as software. 🤷🏻♂️