Some recipes suggest the use a variety of spices in cheese fondue. The only one I use is pepper from the mill.
Salt: normally not needed, if you choose your cheese properly, it is salty enough. Use salt with parsimony, only if the chosen cheese is really mild.
Nutmeg: it is possible to grate some in the fondue if you want, I do not use it.
Sodium bicarbonate: useless.
There is a variety of cheese fondues with mushrooms, in particular boletus (porcini) or chanterelles, tomatoes, or hot peppers. I tried a few, but I always return to the traditional cheese fondue, it is definitely the best!
Boletus and chanterelles in the foreground