End of war. Removal of the army from Ukraine except for Crimea, at first and international oversight of referenda for the regions. Potential threat of cost of reparations and what it would mean for Russia to lose is probably what keeps Russia fighting, and away from the negotiation table.
With hindsight ww2 could have been avoided if it wasn't for the prolonged punishment for ww1.
Russia has too many ways to ruin the world for it to be worth not trying out best to bring them into the fold tightly and fast.
Sadly this will upset Ukrainians but that's unfortunately not as important as avoiding another world war.
Heck as I spell out below, what would happen if Russia simply continues to use its own fossil fuel supply? That's bad enough.
But enacting a Marshall plan requires a definite military defeat first. Else it's pacifying the aggressor again, with the same efficacy as in 1938.
Preventing WWI is a different matter entirely though. It was a very bloody lesson that seriously changes Western mentality; I suspect quite some reasons from 2022 just would not be understood in 1912.
In terms of world peace, I fear the best we can hope for is a prolonged stalemate, or something unexpectedly changing in Russian politics. In the past, I might have hoped that Ukraine would trade invaded land for NATO membership, but it seems this is untenable for both Russia and Europe (e.g. Hungary).
Also, leaving a country to die would surely cause mass political unrest across Europe.
With hindsight most of WW2 could have been avoided if UK/France invaded Germany while its army was busy in Poland. Even during the Sudeten crisis, though this would have probably required a soviet intervention which would have been problematic. Then again the German Army wasn’t that well prepared for an all-out war back in 1938 and might have been bogged down in Czechoslovakia similarly to how Russia is now stuck in Ukraine. Support for Hitler in Germany wasn’t that necessarily that strong prior to the conquest of France especially amongst the military leadership* so a coup wouldn’t have been out of the question. The Battle of France pretty much turned Hitler into a German Napoleon.
* mainly because they didn’t believe that a quick victory was an option and feared a repeat of WW2. They obviously wanted to win a war against France eventually.