Fix all the bugs
Support more formats
Nothing else. Nothing else at all.
Eg Calc should remember the number when changing mode, and do all arithmetic correctly to high decimal precision (not high binary precision).
Notepad should read and write every text-only format known to humankind. (Every codepage of ASCII, UTF-8, UTF-16 etc, EOL, CRLF, CR, LF, LFCR etc)
Paint should load and save every image format known to humankind.
Between those two, you'll keep busy and won't break anything.