Parquet is not plain text (but that is an oxymoron anyways), but otherwise afaik it is pretty solid data interchange format supported by a number of tools already. It doesn't have native formatting system, but it does allow arbitrary key-value metadata on columns so surely something could be cooked up with that.

I've been thinking about a project exactly to replace this sort of Excel misuse. The frontend should be about as Excel-like as possible, where you can just easily type (or import) as many rows and columns you like, but without the formulas and other real spreadsheet calculations. Instead, make it easy to add data types, validation for those types, foreign keys, etc, and store the whole thing in a database.

Yeah, that's pretty similar to what I want. The other feature I'd love is the ability to add "Formatting rows" which are rich text and are marked as not being part of the data. They're merely decorative. I find such formatting is often considered essential for client-facing spreadsheets (somewhat understandably so they know what's going on), but it makes it quite messy to extract data from the sheet.

Sounds like MS Access.

I could even imagine some formula support, but only strongly typed, i.e. SUM only on numerical cells. For a small overview this would still be fine.

I'd quite like these files to be openable in simple viewer/editor software that might not support formulas. I think once you support formulas you open pandora's box with regard to which formulas are supported.

UTF-8 CSV with a few HTML tags for formatting?

You could call it CHU.

Now you just need to make Excel accept it.

I'd like it to have support for things like "this is a <format> cell", where format is something like text, numeric, date, currency. I'm not sure how you'd specify that in HTML.

> text, numeric, date

I think it's already defined?

<input type="text">, <input type="number">, <input type="date">

> currency

You probably need to use the microdata format https://schema.org/Offer