We're really trying two things here - getting Linux to use some Rust, and getting Linux to use some other language, with an independent compiler. That second part is pretty significant by itself (we've had discussions about build systems, compiler ABI bugs, etc.), and even if we were just trying to add C Except Different, it would be a major project on its own.
Once we've figured out how to make them interoperate and what the ground rules are (architecture compatibility, where new development goes, whether the new language can be mandatory, release timeframes, etc.), a third language will be on much better-explored territory.
(I do actually think that Rust is pretty close to ideal for the Linux kernel as it currently exists, though! As I argue in https://ldpreload.com/p/kernel-modules-in-rust-lssna2019.pdf / https://www.youtube.com/watch?v=RyY01fRyGhM , a lot of Rust constructs happen to match idioms and abstractions that the kernel uses with C. But yes, there's a lot of research on both good kernels and good programming languages yet to be done.)