There are already plenty of such languages, Zig among others.
zig doesn't "extend" C in the way that C++ does.
But C++ does not extend C as well.
i guess i should say "the way that C++ _did_", although i do want to say "you know what i mean" since they do still share quite a bit
C3?
Nope, they went their own way now, see recent blog posts.