BBC BASIC is an absolutely superb language – fabulously well structured and flexible – and if a good port were made of it for the MEGA65 it'd turn it into a programmer's dream machine.
I know BBC BASIC from my days with the Archimedes and now with RISC OS on the Raspberry. It's a really great BASIC.
But BBC BASIC would be no reason for me to buy a MEGA65. If I want to program BBC BASIC, then I switch on my Raspberry with RISC OS (for less than 100 EUR) and I can use the most modern variant of BBC BASIC.
But to be able to use BASIC 10, I need a C65 or MEGA65. I would prefer a completed and "modernized" BASIC 10, expanded "in the style of Commodore".
A Commodore computer without Commodore BASIC is simply not a Commodore. And that's how I see the MEGA65, even if it doesn't come from Commodore, of course.
If a new programming language for the MEGA65 at all, then I would probably rather break away from BASIC and implement a different, really modern programming language.