there is a lot of interesting roads, but try to conect line from Slovenia through Predil pass or Kranjska gora, then along Soca river in Slovenia ( go on Mangart mountain road nearby Predil ), then through Istria in Croatia to Rijeka, and along the coast all the way to Dubrovnik. If weather allows you try to climb on Velebit mountain ( several interesting roads and nature realy worth seeing, combination of asphalt and good macadam ) and also go to Biokovo road up to top of the mountain, it is asphalt road. From top of mountain you can see half of mediterian islands and sea. You can stop in several Parks of Nature in Croatia: North Velebit, Paklenica, Krka river, all of them are along your way ). Continue to Montenegro, small country with twisty and curvy roads, and than toward Serbia via Podgorica and Prijepolje. For further ask someone from their Community. If you have enough time, visit some of bigger croatian islands, ferry lines are widely spread.
Depends on time to spend, you can make very interesting vacation in mentioned area.