RFCs for some Stdlib additions are overkill. Just don’t add all of them in one PR so that they can be discussed independently. One PR per Stdlib file would be nice.
If you have some bigger ideas (like how to implement a good private-by-default), just drop them on the forum first (we have a tag for that). Technically we also have a RFCs repository but we are not religious about putting everything there (and you dislike GitHub anyways).
For everything else (smaller feature requests or bug reports) as usual just create a GitHub issue.