New unstable features can be added and approved without going through a Libs FCP. There should be some buy-in from Libs that a feature is desirable and likely to be stabilized at some point before landing though.
If you're not sure, open an issue against
rust-lang/rust first suggesting the feature before developing it.
All public items in the standard library need a
#[unstable] attribute on them. When a feature is first added, it gets a
Before a new feature is merged, those
#[unstable] attributes need to be linked to a tracking issue.