For example the ones from #550 and serde-rs/serde-rs.github.io#22. These don't need to be in serde itself but if there are ones that people ask for over and over, we can stick those in a helper crate and provide a better out-of-the-box experience compared to "implement your own serialize_with" or "paste this code into your project."
Another one: include struct name when serializing struct #554.
Another deserialize_with: a generic comma-separated list of FromStr values #581.
#415 / #636, tagged enums.
#661 - as_base64 for AsRef<[u8]> and from_base64 for From<&[u8]>