Since the US/NATO proxy war in the Ukraine is (necessarily) existential for Russia and (optionally) existential for Our Betters in the DC Swamp, the risk of global thermonuclear war becomes significant if either side seriously feels it is losing.
If cooler heads prevail, then clearly any negotiated settlement has to accommodate Russia’s often stated (and entirely reasonable) desire not to have an armed aggressor directly on its border. That suggests the rational end to the US/NATO proxy war would be a UN mandate to China to put a large military peace-keeping force into de-militarized Rump Ukraine, with the twin objectives of ensuring Russia stays out and US/NATO stay out. Further, China is about the only country which could afford to rebuild Rump Ukraine as a component in its Belt & Road project.
The fun part about this solution would be watching the European anguish. After having huffed & puffed about the supposed threat from Russia on their borders, Euros would then have to live with the mighty PLA on their borders. However, no-one cares about European anguish.