Bitcoin's HD (Hierarchical Deterministic) Keys as described in BIP32 allow for a master key to be created (a private key and a chain code) such that a tree of both public and private keys can be derived from a master key, each which can have additional keys derived from it. In addition, some of hierarchy can be 'hardened' to prevent further children from being created.
Bitcoin's HD Keys allow for some fairly powerful functionality in Bitcoin and address a number of important use cases, including key storage UX innovations, auditability, etc. There are also related schemes such as 'pay to contract' that use similar cryptographic approaches that HD keys use.
However, Bitcoin's curve (secp256k1) is not considered to be fully 'safe'.
The most popular safe curve 25519 unfortunately requires secret keys to have specific bits to be set and unset, making them incompatible with the linearity that BIP32 and related schemes (pay to contract) require.
Are any of the other 'safe' curves linear and thus allow for HD Key functionality?