If they could come together as one they would have had a very decent shot in several tournaments
1992 - Yugoslavia won their qualifying group. Because of the war the team was disqualified. Thus Denmark took their place in the tournament and won the whole thing.
1998 - Croatia got bronze. Add Mijatovic, Mihajlovic, Savicevic, Jugovic, Stojkovic, Milosovic, Kovacevic, Stankovic and that team has enough to beat both Brazil and France.
2018 - Croatia got silver. Add Matic, Milinkovic-Savic, Kolarov, Tadic, Mitrovic, Ivanovic and that team at least gets closer to France.
And that's mostly Serbia and perhaps some Montenegro in there (played together for a while before splitting around 200*s).
Also imagine adding some of these guys to the team Pandev (Macedonia), Dzeko, Misimovic, Ibisevic, Salihovic, Begovic (Bosnia), Jovetic, Vucinic, Savic (Montenegro), Zahovic, Ilicic (Slovenia), Cana (Albania).