[zzo38computer]

I would want a "zoned spreadsheet", where you can define a zone grid on each page; each row and column of the zone grid defines the number of rows and columns of data in that zone row/column as well as names for rows and columns, and each cell of the zone grid (which is named by the combination of the row zone name and column zone name) has properties such as formatting, protection, default values, etc. Formulas can go in zone cells and in data cells (although if a zone cell contains a formula, then the data cells in that zone will have the data automatically filled in and you cannot change it without removing the formula from the zone).

Another thing I wanted is to be able to write extensions in C, and to work with non-Unicode text.

[ithkuil]

Excel "tables" feature is almost what you describe as zones.

W.r.t non-unicode text: what's your use case?

[zzo38computer]

I am not familiar with Excel "tables" feature, so I do not know how similar than what I intended. Is there a better explanation, available, and do other programs (hopefully, which are FOSS) implement any similar features?

Non-Unicode text, I want to be able to deal with text regardless of the encoding and character sets (although appropriate fonts will be needed in order to display it), without needing to conversion, so that any byte sequences can be used even if it is not a subset of Unicode.

[oulipo]

extensions in WASM might probably be easier for portability

[zzo38computer]

I would want to be able to use native code though (with all of the access that is possible with native code); of course such native code extensions would not be possible to add to a spreadsheet file and you must add them to a configuration file instead (both for security and to avoid unportability).

(It does not necessarily mean that WASM is not also possible. If it is, then some kinds of native extensions might be possible to convert to WASM if you want the portability and security that it involves.)