you know what we need even less than another json alternative? another math-language alternative. more so than any other formal(ish) language, a representation (for math) benefits from network effects (because math is both older and broader than any/every other such language).
This is for authoring math expressions which compiles to MathML. MathML has the explicit design goal of being hard to write, and is much more meant to be a compiler target rather than for humans to write. This space is pretty dominated by LaTeX whom almost all professionals use, but is pretty verbose and not exactly easy to use for beginners (or even non-frequent users). Other than LaTeX we have Office Math, which is…
On specific tools I've also liked https://www.imatheq.com/imatheq/com/imatheq/math-equation-ed... for quick casual use since it's just a webpage but still a decent GUI editor.