I’d say, if rocket-* is for official rocket crates, and rocket insists on publishing on crates.io, then you should use *-for-rocket for unofficial crates on crates.io.
Personally, I don’t want namespacing, I want federation. I want projects to run their own repos on their own domains, and I want crates.io to fetch and cache those repos, and anyone using those repos can safely publish on crates.io or anywhere else and rest assured that their crate will be available even if third-party repos go down, because those repos will be cached on the same server you got the crate from.
This is, effectively, DNS-based namespacing, but it’s also decentralized/federated, as each DNS owner can control/moderate their repos however they like. This would make the Rust/Cargo ecosystem extremely censorship-resistant (altho it won’t do much on the community end), and heavily benefit big crates like rocket, diesel, gtk-rs, etc.