Realistically, the area Russia has taken from Ukraine, they are not just going to give back.
Russia began this war occupying Crimea, Donetsk and Luhansk, since 2014 while Obama was president.
Aand since March 2022 under president Biden, taking two more provinces connecting them to Crimea. Since these areas held since 2014 are over 70% Russian, I doubt Ukraine could re-take or hold them, and given these provinces' strong solidarity with Russia, that might possibly be a reasonable concession to Russia, to negotiate these 3 provinces away to Russia, on the condition Russia gives Kherson and Zaporizhia provinces back to Ukraine.
Prior to 1954, Crimea had been part of Russia already (territorially shifted by Nikita Kruschev in 1954 to be part of Ukraine, but all still within the Soviet Union till 1991), and at the time of invasion in 2014 , Crimea was already 90% ehnically Russian.

https://blogs-images.forbes.com/tatianaserafin/files/2015/03/Map.png

https://www.geoawesome.com/wp-content/uploads/2022/03/Ukraine_LeftAffixedMaps_2-2.webp

https://images.squarespace-cdn.com/...ussian+as+their+unique+mother+tongue.png



https://euromaidanpress.com/wp-content/uploads/2014/09/plurality.png

https://blog.uvm.edu/aivakhiv-ukrta...guagesCensus2001detailed-en-1024x704.png

https://www.brookings.edu/articles/10-maps-that-explain-ukraines-struggle-for-independence/

( BBC: Tracking the Ukraine/Russia war in maps over 3 years )
https://www.bbc.com/news/world-europe-60506682

Although even after Russian invasion of Crimea, in Donetsk and Luhansk a majority even in those two provinces still favored being part of Ukraine independence over the prospect of uniting with Russia.
So I think ultimately, a popular vote of Ukrainians in those three provinces should determine whether or not Russia is allowed to keep them. And if it were up to the populations in those three regions in a vote, I think only Crimea would choose to be part of Russia.