Rather than an orange slice, I think it's better to cut a strip of peel, rub it around the inside of the glass, twist and put it in the drink. This gets you the orange oil without clouding the drink or having juice that changes the flavor. Triple Sec or something similar would be nice, too.
My favorite variant of a standard Old Fashioned is a banana rum Old Fashioned, which I found here:
https://www.foodandwine.com/recipes/banana-rum-old-fashioned
I had never heard of a Wisconsin-style brandy Old Fashioned until I went to the Hob Nob, an old-school supper club in Racine, WI, for the first time a few years ago.