Same here, if there would be a possibility to use the Commodore brand, it would be cool to call it simply Commodore C65, or something that is VERY close to that.
On the other hand, it's understandable that a distinction from the original C65 prototype would be handy. In this regard, I think something like C65+ would actually be not a bad idea. Or C65 Plus. It would be along the lines of the Amiga 500 Plus, and it would be cool to have a name that even Commodore might have used themselves.
In my opinion, it should always start with C65. Not something like M65, that doesn't sound like a Commodore.