I could see the outline of a potential deal along the lines of:
- Donbass stays part of Ukraine but with Catalunya-style regional autonomy, including as to language
- Crimea stays with Russia subject to a confirmatory referendum and, in any event, a long-term lease of Sevastopol
- Ukraine moves towards EU accession
- Ukraine stays neutral for military alliance purposes but NATO (or US, UK, France, Germany, Poland) and Russia jointly guarantee the territorial integrity of Ukraine
That sounds roughly fair (not perfect) and allows Ukraine, the West and Russia each to spin it as relative success.
However, the problem is that Putin has destroyed any remaining vestige of credibility and it seems the West (even Germany) has finally moved past the point where they see any possibility of a working relationship with him. The West can continue to try squeeze Russia to encourage a palace coup in Moscow but, in the meantime, it’s Ukraine in the meat grinder as its citizens get killed or forced out and its infrastructure gets smashed up. There is no easy answer right now unfortunately.