A sustainable solution for Russia would begin with decentralisation, which would respect the strong autonomy of the areas of Eastern Ukraine that are now part of the so-called Donetsk and Lugansk People Republics, of course with the understanding that they remain an integral part of Ukraine.
Second is what I call the Finlandisation of Ukraine. This term could be understood differently, but it does not necessarily mean a bad experience, it may be very positive in a security and economic sense. My reading of Finlandisation is that Ukraine should not be part of any military alliance and it should be legally binding. The Ukrainian people should also be free to decide which union or formation to join. I am personally sceptical, however, about whether the EU is currently so interested in accepting Ukraine as a member. But I do think Ukraine needs significant assistance in rebuilding itself after the civil war. Russia should not be against a joint effort between the EU and a Eurasian Union on the restoration of Ukraine. In addition, the issue of any attempts to blockade Crimea would certainly not be acceptable.