I gotta say, I think it would be in Ukraine's interest to have a vote in Crimea and the Donuts and Loserchance Oblasts subregion by subregion as to if they want to stay or go with rules to prevent enclaves and excessive bordergore. I mean a real election, not that fake bullshit done by Poutine. A real election watched over by the Swiss, the Liechtensteiners, People with a rep for neutrality and not being corrupt.
I don't think it would do anyone good for Ukraine to bring back in a bunch of people who would rather be Russian, if after almost 10 years and the war and their peeps being called up for cannon fodder they still want to stay, fuck em. If Ukraine takes it back it will forever be a sore point with Russia, at least with an election you can't bitch too much about it and also having people in your country that want to fuck your shit up isn't a good thing either both in general and with elections.
Give people a chance after the election to move out, easy peasy sort of lemon squeezy.
Again by subregion, no enclaves. If Quebec North wants to stay in the Canadas fine, if Westmount wants to but the rest of Montreal didn't, well, that sucks.
Also that would be a proposal that would be very hard for Russia to turn down, sure they could easily, but they would look like even greater shitstains on the global opinion then they do now.
However Ukraine should definitely be allowed to keep all them frozen assets because fuck Poutine and Russia for being dicks, they gots ta pay for what they did. Also they lose all their no NATO rights as far as Ukraine is concerned.
That would be both the minimum and maximum I'd accept.