Very cool @caubin! Exactly what I had in mind for this too 
Do you already have a good list of predefined existing queries and actions? From the screenshots, I don’t think so.
I think you could easily take the majority of snippets from http://snippets.xwiki.org/xwiki/bin/view/Extension/ and add them to this gardening app. For example:
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Deleted%20Documents
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Delete%20Attachment
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Delete%20Space
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Delete%20all%20users%20in%20current%20wiki
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Delete%20Revisions%20From%20Author
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Remove%20Objects/
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Remove%20inactive%20users
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Remove%20All%20Anonymous%20Comments
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Delete%20Spam%20Users
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Remove%20All%20Comments
- http://snippets.xwiki.org/xwiki/bin/view/Extension/Set%20rights%20to%20imported%20users/
- etc