Yeah that will be a tough one. Crimea has been only been de jour part of Ukraine since 2014, while it's been de facto part of Russia. They will not abandon it. And aside from what Putin and the Kremlin might accept, the people there are also primarily ethnic Russians who have had 8 years media control and Kremlin propaganda by now. So I'd stick my neck out and say they will not be very welcoming to a return to Ukraine any time soon.
I'm not here to tell the Ukrainians when to stop fighting and what deal they should take, but if it were me I think a deal along the following lines would be quite appealing:
- Recognition of Crimea as Russian territory
- More autonomy and local powers to Donbas while remaining part of Ukraine (disarmament of the rebels there and removal of Russian "little green men")
- Some superficial "de-Nazification" for Russian eyes mostly (disband the successor units to Azov battalion etc.)
- No NATO membership, but EU membership within Ukrainian rights
- No change of regime in Kyiv or any commitment to have any Kremlin stooges there in any executive role. Complete political independence.
- Right for Ukraine to arm (but no nukes)
While Ukraine has fought the Russians to a standstill it's still worth remembering that it's their country being invaded, their cities being pummelled, their civilians getting killed. They will not only have to put up with that for weeks/months, but they'll have to go on the offensive to get Crimea back. In which case the situation will reverse because going on the offensive, as the Russians have found, is a
lot harder than holding ground and defending with ambushes etc. Only the Ukrainians themselves can tell if the prize (Crimea) is worth the fight.