A "fork" is really discouraged, IMHO.
In fact, the idea behind ZX BASIC is people to contribute to the compiler deveolping *backends*. ZX Basic is a retargeable compiler, which means the Source code is translated to Intermediate Code and from there to asm and finally assembled. Everything is modularized, so you can (or should?) add a different module on each stage for different purposes. E.g.
- If you change the backend, your program will compile for another platform.
- If you change the fronted, you can create programs in another language (e.g. C, or Fortran).
Also, forking the compiler could eventually lead to different language flavours or variations (one for NES, another for ZX Spectrum). Even though INK or PAPER might make nonsense in other platforms, new features can be added just by using function calls and subs (this is what C does, in fact). So we need not to change the language adding incompatible features between platforms (incompatible functions are OK).
If fact, NES is an 8 bit CPU, so the backend should be not very hard.
Well, that my opinion. ZX Basic is licensed GPL, so if people want a fork, they could proceed as they want.