Why would you like to use an on camera flash from an artistic perspective?
Nevertheless, any fully manual modern era flash without proprietary pins would work fine. As with all digital cameras, do not stick a very old film era flash on it as it will fry your camera.
Edit: now that I think of it, I think Metz might have made a TTL flash for the DF platform.
Personally, I have only ever used mine with radio triggers and that works like a charm.