My point was not to make UN the policeman, but to get countries to agree to change the founding principles of UN (which is exactly the sovereignity you mention). The cost involved should be big enough to discourage everyone and their uncle to claim a new state, obligations of signing all the UN resolutions and declarations should be present. Basically, I think it would discourage many smart people from wanting to separate in the first place: eg. if Catalonia decided to secede, they'd have to be willing to accept Barcelona to secede as well, and suddenly that is not as tempting economically.

FWIW, I am a Serbian myself, and the Serbian people have been at both ends of the bargain: they've stopped from seceding from Croatian and Bosnian territories for UN border reasons, but Kosovars are also not fully separated from Serbia because of UN ruled (though de facto both Bosnian Serbs and Kosovars have separate "states"). I am convinced that if such framework existed prior to 90s, we'd see much fewer wars (if any), since nationalists wouldn't have a way to rise to power. I don't even think we'd see much splitting up of Yugoslavia even, or at least not in any meaningful sense (maybe all states would have joined EU as independent states at the same time to make it EUgoslavia :).

I haven't worked this out in detail, and I am sure there's a bit of psychology to deal with too (people do need a bit of us vs them, which we are trying to substitute with sports), but I am sure that if we had a bunch of smart people on finding a way to make seccesion fair, we'd at least get a reduction of violent conflicts.