JVCF is a JSON schema for describing genetic variants. BioJSON is a JSON schema for multiple sequence alignments. The output of mygene.info's gene annotation web service is JSON-formatted.
You might build one of your own schemas from any or all of these, as inspiration for structure and content.
Some tabular results can be hierarchical, but use inefficiencies in field values to apply hierarchy. JSON is inefficient for storing tabular data, but it is absolutely wonderful for hierarchical data. This difference is perhaps worth considering.
One could think of storing a gene as a master object containing various haplotypes, exons, introns, features, associated variants, associated TF binding sites, gene ontologies, etc. Ordering or prioritization of such items would be possible with the use of sorted lists within a gene object. The format is as extensible as it is a matter of adding a property, list, or object to the schema.
To gain adoption or "buy-in", build tools which publish it to users, and tools that consume what users get in order to do things, along with conversion and validation utilities.