Well we've implemented a GraphQL API for Bonfire, which is a much more flexible way for frontend apps to get exactly the data they need and provide a richer experience, so hopefully some alternative frontends and mobile apps will want to implement that at some point (maybe we can even standardise on a common GraphQL schema to be use by different federated platforms!)... The other option would be that we/someone implements the Mastodon REST API as a Bonfire extension.