Add an option for alternative database formats. json may be most readable, but a SQL database would be more efficient.

SQLite would be preferable if going down that route, as it is file based rather than server based.
Regardless minecraft may not scale well enough with high playercounts to even benefit from going fancier than .json. Are there any existing mods that use SQL / SQLite?

If you're asking specifically about Forge mods, I'm not sure. I know LuckPerms has options for quite a few storage types, including SQLite and H2 for local databases, as well as multiple remote databases and config file based storage systems.