The Zagreb idea is a good one. You could spend several days there. But I think it would be difficult to fit DBV, Zagreb, and SPU (Split) all into an 8 night itinerary.

I'd recommend either sticking with the coast or do Dubrovnik (and surrounds) & Zagreb. I didn't mention Split in my earlier post, but we did enjoy our stay there, more than Dubrovnik if I had to choose. Our last trip was basically from SPU to DBV by way of the islands. But we had about 11 nights if I remember correctly.