I mean, the existing framework would be the United Nations. Doesn’t have to be, but it’d have to be one where the permanent members don’t just reject the legitimacy and jurisdiction of international courts to enable invading countries. Those courts should be able to review and perhaps rule on such things like border issues and separatist movements for both standing and substance.
Pragmatically speaking I would much rather borders change via things like Brexit, Kosovo (after 1999 when UN intervened), and the Crimean referendum vs Donbass-Donetsk, Kosovo (before 1999), or Gaza.
The above are all complex issues with varied and unfavorable outcomes but the point being there is a very clear dividing line.