There’s a rather heretic but interesting hardware project for the C64: transplanting a Hitachi 6309 for the venerable 6510 CPU. The Hitachi 6309 is an enhanced CMOS version of the Motorola 6809, providing a 6809 emulation mode and a native mode with an enhanced feature set (like additional registers, 32-bit arithmetic, division, etc).
Notably, there is OS-9, especially the NitrOS-9 variant with support for the 6309.
The prototype runs a test Kernal ROM and executes sample code, including a Mandelbrot set generator. IRQ and VIC-II raster interrupts work, and file loading functions on an NTSC C64 with a 1541 drive. However, PAL systems experience loading failures, and enabling sprites causes issues. Development is based on an NTSC C64 (board revision 250407), though other revisions might work with adjustments.
If this evolves into a solution with switchable 6510/6309 personalities, this could be quite interesting…