I see you have probably translated my previous message, but let's write it again in English so I can be sure you understood me well. So, I think the route from the begining of this post is not so good as it pass many nice cities. If you pass all cities, you are going to see just fucked Balkans roads
Serbian people is famous upon hospitality, so you should not miss that and Belgrade is very interesting city anyway. Respecting your route, I would suggest not to miss these cities: Sarajevo, Mostar, Dubrovnik, Thessaloniki, Bucharest. But if you modify the route a little bit, I would suggest Belgrade for sure.
You should maybe skip Montenegro and Albania and go from Dubrovnik to Belgrade:
https://www.google.de/maps/dir/Dubrovnik,+Kroatien/Belgrad,+Serbien/@43.7462693,19.3529265,8z/data=!4m13!4m12!1m5!1m1!1s0x134b8ba20835e87d:0x400ad50862bd500!2m2!1d18.0944238!2d42.6506606!1m5!1m1!1s0x475a7aa3d7b53fbd:0x1db8645cf2177ee4!2m2!1d20.4489216!2d44.786568?hl=de
Or you can visit Montenegro as well:
https://www.google.de/maps/dir/Dubrovnik,+Kroatien/Podgorica,+Montenegro/Belgrad,+Serbien/@43.5124491,19.3225709,8z/data=!4m20!4m19!1m5!1m1!1s0x134b8ba20835e87d:0x400ad50862bd500!2m2!1d18.0944238!2d42.6506606!1m5!1m1!1s0x134de8079606867d:0x6bf78a76ea588ae9!2m2!1d19.262892!2d42.441286!1m5!1m1!1s0x475a7aa3d7b53fbd:0x1db8645cf2177ee4!2m2!1d20.4489216!2d44.786568!3e0?hl=de
It depends what is the primary goal of this trip. All the best!