You'll have to look for it but about a year ago we had a long thread about the history of four cylinder Ducati's, I know at the time I even found and added some pics of the Bi-Pantah.
Personally I'd vote for the BiPantah to resurrect the SS category, with fuel injection you could have a classic styled aircooled motor with huge horsepower numbers. Make it minimally faired like the 70's supersports were, nice....
The Vyper is addressing a question most Ducati people never ask. And if you wanted an Italian Cruiser Guzzi does it best.
The apollo is just a little too far back to resurrect IMO.