This is not a replacement to opensearch implementation which is useful to end users.

Add ability to invoke search using bookmarks.
Keyword Search allow me to:
- Sort Keyword Search entries by categories; and
- Manage a larger amount (even hundreds) of search providers.
http://kb.mozillazine.org/Using_keyword_searches

Example:
Titile: KDE Bug
URL: https://bugs.kde.org/buglist.cgi?quicksearch=%s&no_redirect=1
Keyword: bugkde
> bugkde postmarketos
> https://bugs.kde.org/buglist.cgi?quicksearch=postmarketos&no_redirect=1

The following is from my personal view only, thinking of how to implement this feature in the most efficient manner.

Storing data:
Keyword search bookmarks to be stored under the table keyword_search (not search_engines) of browsedata.db and inside bookmarks.json file.

Detection process:
Upon bookmarks import and upon clicking OK (for a single bookmark), Falkon will check whether URL contains the sign %s.

Exporting data:
keyword_search should not be exportable because it is resulted from bookmarks.json.