I would like to see an option to display results in KRunner (and maybe in general Milou-based searches) not grouped by category, but sorted strictly by relevance, as in the application launcher and application dashboard.   

Grouping results by category may make it easier to identify of what type a result is (though that is also provided the icons), but it also makes the most visible results much less relevant: A very-but-not-the-most relevant result will be outranked by any somewhat-but-only-slightly relevant results that happen to be in the same category as the most relevant search result. 

For example, I often run into the situation that the result I am interested in is a VSCode project (from the VSCode project runner extension), there is a closely matching recent file, in addition several other vaguely related recent files, and the closely matching recent file is considered equally or slightly more relevant than the VSCode project, and wins over it. In this case, the very relevant but not maximally so VSCode project result I am interested in will be buried below lots of rather  not so relevant recent files, simply because they were lucky to be in the same category as the winning search result.

I have found that I find it easier to quickly find what I'm looking for in the search provided by the application launcher, but prefer the standalone popup UI of the KRunner application. I think it would be useful to have an option to combine the benefits of the two UIs.