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, as well as since March 2022 taking the two more provinces connecting them to Crimea. Since these areas 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 negottiae these 3 provinces away to Russia, on the condition Russia gives Kherson and Zaporizhia back to Ukraine.
Prior to 1954, Crimea had been part of Russia already, and at the time of invasion in 2014 , Crimea was already 90% 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 two provinces should determine whether or not Russia is allowed to keep them.