If there is only one match in the search results - then launch it automatically + add option for toggling such functionality.
I don't see how this can result in predictable behavior or even be helpful. The results change as-you-type and you could end up with more than one result eventually. Also, runners are queried asynchronously and some takes longer than others to complete, so there usually is only a few results at first and then it fills up. Furthermore, some runners don't result in "executable" results, like the calculation or unit converter results, etc.