Many of them would actually be handled just fine as CSV files (as fine as any random actual CSV file), but I use a different string syntax ([like this] instead

Many of them would actually be handled just fine as CSV files (as fine as any random actual CSV file), but I use a different string syntax ([like this] instead of "like this"), so that would pose some problems. It would not be hard to write an Excel plugin to read my format, while restricting to a fixed set of basic datatypes, and getting some usability gains there. The problem I see is that full-featured interoperability is hard to achieve because the set of supported features is hard to agree on.