There are people around making both acoustic and electric fiddles with - five strings (low C so it's like a viola and a violin) - six strings (even lower, bottom F!) - octave fiddles (an octave lower than normal) ... and probably other combinations, for all I know.
I think viola strings work well, but the experience of the fiddler in my band (who plays a Violectra 5 string) is that the bottom string feels a bit soggy if you are used to a normal violin. The six string one must be a bit of a joke, similarly the bottom end of an octave fiddle.