A fruit cake would not be sugar free strictly speaking, since fruits have sugars, but you can make a fruit cake without adding extra sugar by using lots of very sweet dried fruits like dates, raisins and figs.
A cafe I know has a "famous" vegan no sugar no oil cake which is made of about 50% mixed dried fruits. They use dried dates, raisins and ...
Here is a recipe I use.
2 to 3 shakes from each of the following
(in powdered form from spice containers that have small holes on top):
Just leave off the Cayenne for milder spice.
I am going to try hops leaves, simply because someone in my neighborhood has hops plants they don't want, and I don't make beer but I see that the leaves look very similar to grape leaves, and the plant is edible.