Allow functions to return values
One of the most common gripes for me when using CMake is the inability to use a result of one function call as an argument for another one without storing it into a variable (even if it will only end up being used once). It would be nice if CMake could be extended to allow return values instead of having to use output argument. Especially since most functions return just a single value anyway.
Someone actually seems to have made a CMake fork that supports this recently:
https://oleksandrkvl.github.io/2020/08/09/allowing-cmake-functions-to-return-value.html