It seems people pretty universally dislike type annotations and overly verbose comments, like Ruby's YARD or Java's Javadoc. Also, if your new language doesn't compile with a standard C compiler, kernel usage is probably DOA. That means you want to keep the source code pure C and store additional data in an additional file. That additional file would then contain stuff like pointer type annotations, object lifecycle and lifetime hints, compile-time eval hints, and stuff to make the macros type safe. Ideally, your tool can then use the C code and the sidecar file together to prove that the C code is bug-free and that pointers are handled correctly. That would make your language as safe as Rust to use.
The hardcore C kernel folks can then just look at the C code and be happy. And you and your users use a special IDE to modify the C code and the sidecar file simultaneously, which unlocks all the additional language features. But as soon as you hit save, the editor converts its internal representation back into plain C code. That means, technically, the sidecar file and your IDE are a fancy way of transpiling from whatever you come up with to pure C.