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In this post I will say few words about current situation, then I will describe two cases in which current behavior is suboptimal and finally I will try to give some proposals what can be done about it.

When Nepomuk file indexer is turned on, and KRunner Nepomuk plugin is turned on (which I believe is by default), user can start typing file name or content into KRunner and get results of search. This is quick and convenient way to open files from hard drive without really wondering about their placement in file system tree.

User case #1
I have made some image and put related metadata into that file. When I search for that metadata, KRunner will show that file for me. But do I want to view that file? Or do I want to make quick change with KolourPaint? Or do I want to go into full-time file editing with GIMP or Krita? Or maybe I want to e-mail that image to my friend?
All these situation may happen, yet there is no way for KRunner to decide what user wants and what to do. Default action is viewing image in Gwenview, which will work for most cases, but may be tiring if user wants to edit images often. User have to open file in viewer and from it select "Open in..." another program.

User case #2
I remember part of file content and I want to open another file, which I am sure is in the same directory. E.g. I have directory named "Book" somewhere deep inside file system tree, in which each file is separate chapter. I remember only part of chapter name and I know that I want something from another chapter.
In this case, "Open containing folder" action would be the best. Right now user has to open file (most likely in Okular or Office suite), select file properties, copy path to directory and open it in file manager.

What can be done about it?
First and most obvious proposal is support of right mouse button on search results. In that context menu we could have "Open containing folder", "Open with Gwenview", "Open with GIMP", "Open with Okular" and similar actions.
Another way is to show little wrench icon next to hovered search result, the same way as KRunner behaves for applications found (most likely by Programs plugin). After clicking that wrench, user could specify what to do with that file this time.
Either way, all backend is already done by .service and .desktop files for file managers. Perhaps we can mostly reuse it, making changes easier.
Of course these are only proposals of UI for that. There may be other, better ways to achieve that. Point of this idea is to show cases, where current behavior is limited.

99.99% of the times I need the default action but I like the idea and I will try to add this feature in Nepoogle.

The second one it's, in my opinion, crucial to manage files with metadata and, for this reason, I implemented this feature in the first version of Nepoogle and even I have a service menu to open file location.