There is actually a drink, similar to an
Alexander, with Scotch instead of Gin and vanilla ice cream instead of heavy cream. They are actually quite good. As with the Alexander, you can substitute the liquor of your choice. I've also had them with cognac (brandy) and Irish whiskey.
I don't think I would care for the juniper taste of gin with Crème de Cacao (dark is best). Dark rum might be good.
You can also substitute other liqueurs. One of the coffee liqueurs (Kahlúa, Kamora, or Tia Maria) or nut flavored liqueurs (Amaretto, Frangelico, or Kahana Royale) would probably work well.