First of all... I don't want to open a can of worms here, and I'm aware that there's great work being done on the OPEN ROMs, and under no circumstances I want to belittle or discourage this work.
Having said that, I thought it maybe worth mentioning that BBC BASIC for the 6502 is available as source code here:
...and what's more, there even has some work been done of porting it to the C64 in the past (there's a C64 "target" in there).
Now I don't want to raise any false hopes... those sources won't build on any modern assembler without lots of work (in their present form, they have to be built on a RISC OS 3.1 host with BASIC65 patches). And all the graphics stuff won't work without someone writing the VDU drivers for it.
But... having worked with BASIC10 and the dev board for two months now (you may check out yaped32, hopalong and lemonade, if you like...;-) and on the original C65 ~20 years ago, I feel I can safely say, there's an urgent need for a reasonably modern programming language on the MEGA65.
BASIC10 surely is interesting from an archaeological point of view... but as a tool to create new programs that make use of the MEGA65's advanced features, it just simply and plainly sucks. Floating-point-only arithmetic, GOTO/GOSUB hell, no named functions and procedures and variable names limited to two characters simply don't belong on such a beautiful machine.
So... just a little food for thought: for an experienced 6502 coder, I think the BBC BASIC sources are definitely some sweet and juicy low hanging fruit. Who knows, maybe someone feels called to have a closer look at it. I think the MEGA65 as a platform would benefit considerably. Maybe it would even be possible to combine OPEN ROMs and a port of BBC BASIC!
P.S.: RISC OS (of which BBC BASIC is now a part) is -- since October 2018 -- released under the Apache 2.0 license.
P.P.S.: There's some information on porting here: http://mdfs.net/Software/BBCBasic/6502/