My thought is one or more of the following:
1) accept annexation of crimea.
2) some sort of special status for the breakaway regions, maybe up to acknowledging their independence.
3) promise not to join NATO.
4) promise some sort of permanent neutral status.
None of those options is great but it seems like the other option is the balls out one where Russia likely conquers Ukraine and then fights a bloody insurgency for years. Maybe in the end Ukraine wins that way but at what cost?
I've been thinking about it more. I don't see how you convince Ukraine to accept that. Additionally, and maybe this doesn't matter especially since it's just my opinion, it seems like an immoral compromise. Giving the bully anything seems like a really bad precident to set.
Of course, it is Ukraine's decision to make, they are the ones suffering and I don't think any rational person would fault them for taking that deal to put a swift end to all this and attempt to get back to normal.
They just don't strike me as a people that would be willing to accept that at this point, and they deserve way better, but how do they achieve it? I've gone over it quite a few times since it became apparent to me on day 2 that Russia has been exposed as a paper tiger. Someone needs to throw Putin a realistic lifeline and it ain't going to be easy and it ain't going to be anything Putin is going to be happy about. Putin is in no position to demand anything, and he needs to be made to see that, if it's at all possible. That's a huge if.
However, if he doesn't recognize it soon this is going to go one of 4 general ways as I can see it, from worst to best case scenario:
1) Nukes fly. Putin knows he can't live with this failure so he decides to take everyone down with him. People around him either try to stop him and fail or are on board because they know they're just as fucked as he is. I don't know the mechanics on how many points of failure there would be to prevent his launch. For all I know it's a keystroke away for him at all times. Maybe there are tons of people that need to turn keys. Doesn't matter, just saying this would be worst case scenario no matter how unlikely or likely it may be.
2) Putin razes Ukraine until every last Russian soldier is dead or captured and Russia is left isolated like NK until Putin is dead.
3) Putin abandons this disaster because he realizes it's not sustainable and he literally cannot accomplish what he set out to do. Russia is still left isolated until Putin is dead, but maybe we gradually lift sanctions to at least allow his people to eat in exchange for leaving Ukraine alone.
4) Putin is removed from power. Either because of his failure or because he tries to launch and is stopped like you think is likely in that scenario or one of a hundred other ways, most of which also result in his death.
That's it. Those are his options without Ukrainian capitulation, which I honestly think is more unlikely than any of the above. He's really painted himself into a corner. 99 out of every 100 ways this can go end with his death, and the last 1 of each 100 end with him living with this humiliation for the rest of whatever life he has.
However, if he CAN be made to face that reality, now the door is cracked to offer him that lifeline that he needs to maybe survive, if that's even what he wants. The only lifeline that makes sense is convincing him that if he wants to live, playtime is over. You've shown the world your military is a fucking joke. You will cease all espionage. If Crimea and/or these other regions of Ukraine slip away from you and return to Ukraine as a result, you will accept that outcome. We will all return to a path of nuclear disarmament. You will allow your country to fairly elect a leader asap. You will take your money and live out the rest of your life in retirement wherever. Russia will pay restitution for every bit of damage, including a whole bunch for the loss of life. In exchange for these concessions we will immediately lift all sanctions in an attempt to salvage Russia's economy before it's too late to be able to even make good on reparations. The second you break any of these conditions all the sanctions slam back into place until you are removed from power by your own people. If we even THINK you've committed more espionage, whether it be poisoning or hacking or whatever else, sanctions until you are dead. There is no further bargaining, take it or leave it.