We would like to see functionality developed to allow a search using the 13 digit form of the ISBN find the matching 10 digit ISBN (and vice versa) when the other length ISBN is not in the record. We accept that if both the 13 digit and 10 digit ISBN were present in the record then the record would be found whether we entered the 10 or the 13 digit ISBN. An example search is 9780500280775, where we expect to find the record with (only) the ISBN of 0500280770 already in the record.
This should be already possible, if you turn on SearchWithISBNVariations and make sure UseQueryParser is off: http://translate.koha-community.org/manual/16.05/en/html-desktop/#SearchWithISBNVariations