RTF works with anything. On XLS and the rest. it should be a common ground like RTF for word processing. And I don't mean CSV/TSV, but a truly open core standard. No, nothing like HTML5 where it grew up organically like a disease, with bloat on top.

Also, input and output shouldn't be allowed to be mixed by design. Data in one sheet, and formulae output in another. And no automatic parsing should be done, ever. Remember Excel with Genomics, or the issues mangling dates. Or worse, locales, which are another issue under Unix.