There are arguments for and against but for colour: D in GCC uses a shared frontend, which means it's not hard to maintain the frontend aspects — but that also means the frontend is a monoculture which is not very healthy at the moment.
If someone will pay you to do it I would do a new frontend — it's surprisingly little work to get something you can bootstrap with.
Having maintained both C++ and D for different compilers, it's way easier to do it in D since the front-end features are the same (modulo builtins) and the stdlib is 99% the same.
If someone will pay you to do it I would do a new frontend — it's surprisingly little work to get something you can bootstrap with.