TOML is fine for configuration, but not an adequate solution for representing arbitrary data.
JSON is a fine data exchange format, but is not particularly human-friendly, and is especially poor for editable content: Lacks comments, multi-line strings, is far too strict about unimportant syntax, etc.
Jsonnet (a derivative of Google's internal configuration language) is very good, but has failed to reach widespread adoption.
Cue is a newer Jsonnet-inspired language that ticks a lot of boxes for me (strict, schema support, human-readable, compact), but has not seen wide adoption.
Protobuf has a JSON-like text format that's friendlier, but I don't think it's widely adopted, and as I recall, it inherits a lot of Protobufisms.
Dhall is interesting, but a bit too complex to replace YAML.
Starlark is a neat language, but has the same problem as Dhall. It's essentially a stripped-down Python.
Amazon Ion [1] is neat, but I've not seen any adoption outside of AWS.
NestedText [2] looks promising, but it's just a Python library.
StrictYAML [3] is a nice attempt at cleaning up YAML. But we need a new language with wide adoption across many popular languages, and this is Python only.
Any others?