I would prefer to have different fonts for the commands (monospace) and the text (semi serif) for better readablility of the text.
As far as I can see this is not possible with the font settigns dialog. Would this be possible in principle ?
Everything is possible in principle ;-) What you are asking is not possible without doing some serious coding, so I have to disappoint you.
Btw, this feature should be implemented in Kate, the editor part. Maybe they think it is a good idea.