Since this machine is surpassing the C65 by far, so imho C64 Mark II or anything alike is out of the question. The name MEGA65 is quite matching, and leaves also space for other similar projects.
Let me give you an example: on the 2600, the clock is divided by 3 by the TIA. Let's say, you build a version with a control register, that lets you drop that divider and clock the CPU three times as fast, include options to run games from cartridge and mass storage alike, you'll get a 2600 on steroids, maybe a MEGA2600, or MEGA26. So if something like this will happen as a successor, there would already be a brand to build upon.
If it can be made it a Commodore MEGA65, it would be fine, if the price is fair. On the other hand, as a compromise, just having a licensed C= key on the keyboard would also be cool, if the name Commodore is too expensive.