Didn't we go all over this when a hostile GPL re-licensing of a (painfully) reverse engineered OpenBSD driver was attempted?
I'm not sure what you could do, that would also be attractive for people to use and contribute to, that would add enough GPL content to make such a thing fly even in theory.
Maybe add a bunch of the GCC backends to LLVM? That's where it's most conspicuously behind GCC. There's also the precedent of GCC derived pre-Clang front ends, although I don't know how many of the non-C and C++ GCC front ends are seriously important (there's Ada, but that's got its own complexities).
I'm not sure what you could do, that would also be attractive for people to use and contribute to, that would add enough GPL content to make such a thing fly even in theory.
Maybe add a bunch of the GCC backends to LLVM? That's where it's most conspicuously behind GCC. There's also the precedent of GCC derived pre-Clang front ends, although I don't know how many of the non-C and C++ GCC front ends are seriously important (there's Ada, but that's got its own complexities).