The Signal protocol is for all intents asymptote the state of the art in the design of a secure messaging protocol. There doesn't seem to be any meaningful improvements to the design without changing the requirements.
New requirements might be
- Post Quantum forward secrecy
- Groups messaging with transcript verification
- Security weakness in x25519 or AES-CBC-HMAC or SHA256 primitives.
If you don't have any new requirements, crypto protocol developer time is a scarce resource. Why reinvent the state of the art?