Sticking with gin/Campari combinations, if you could find St. Germain you could make a Bitter Elder, or with Cointreau a Robert Hess Jasmine. If you like grapefruity flavors you'll likely enjoy either of those.
But you might just keep tinkering with the Negroni too. Are there vermouths one shelf up from what you have that might be worth trying out?
... or a bottle of Suze away from a White Negroni (with the dry vermouth).