ericgus wrote:coco3 mister core already exists (unofficially anyway .. you can google for it, it currently does not work with 128mb ram as it uses the older framework)
Yes, that's Roger Taylor's core and IIUC he's focused on his own hardware platform rather than MiSTer.
Threre's another CoCo3 implementation by Gary Becker that - IIRC - is open source. It is targeted to the old DE1 hardware and supports a popular add-on board for analogue joysticks, more RAM, RTC and WiFi module. I have a couple of these set-ups myself. That might be a more feasible starting point for a MiSTer port if it is indeed (still) open source.
Years ago I had designs on my own implementation after my Coco2 but, like many other projects, got put on the back-burner because there was always something else to do!