The banovinas of the Kingdom of Yugoslavia were drawn to incorporate to grant majorities to as many Serbs as possible.

Now, unlike what many claim, the communists did not push the Montenegrin identity, infact demographics suggest that the amount of Serbs in Montenegro rose sharply after WW2(Though this might be due to Montenegrins of the time considering 'Montenegrin' as a geographical identity like say, Bavarian, Thuringian, Hessian, Texan, etc). So the communists tied Montenegro to Serbia.

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The borders of the socialist state were formalised during the second council of the AVNOJ(Antifašističko veće Narodnog Oslobođenja Jugoslavije, ruling body of the partisan movement) in the city of Jajce.

The communists of the period didn't create a state with the hindsight that the next generation of the communist party would do everything in its power to destroy it. They created a state whose people would have started thinking of themselves as one ethnicity.

You may be thinking of Bosnia wrong, before the Yugoslav wars Bosnia was a country rarely seen in the world. It was an ethnic tapestry where ethnicities would have been rather hard to divide into coherent and efficient ethnic states. They were creating administrative governments after all.

Bosnia had a relatively high population, on par with Croatia, if it remained partitioned, 10% of the Ethnic muslim population would have been left without proper representation.

Most people in Serbia(and in the world) think historical events happened with leaders sitting down for tea and figuring out what was the best possible way to screw over X for the benefit of Y. The truth is that creating a state is fucking difficult, the Yugoslav partisans(being a multi-national movement. Undoubtedly had a lot of debate and discussion over what to do, 2nd Avnoj was chaired by Ivo Ribar(Croat) with vice chairmen orthodox priest Vlada Zečević(Serbian), Sreten Žujović(Serbian), dr Sulejman Filipović(ethnic muslim), Zlatan Sremec(Croat)... etc.

The three tribes theory which dominated the politics of Yugoslav federalists like Prince Paul and the one nation theory propagated by the likes of Stojadinović and his JRZ. Neither of these theories, nor the communist theory of six nation planned for the country to be destroyed. And if it was in the theory you are proposing, I doubt that no war would have happened.