I think that a return to the CFE treaty with the new realities (Eastern Europe is in NATO) resuming Open Skies with Russia being allowed to fly over the US and not just Europe, requiring Ukraine to implement a Minsk 3.0 as written on the threat of complete witholding of westen aid, a guaranteed neutral status for Ukraine and Georgia, international observers as a condition of any autonomous elections in the LDNR, and a new IRBM ban treaty would work. It wouldn't require disarming the Baltics or Poland, and could come with a clause that automatically triggers consequences if Russia intervenes in Ukraine militarily. The biggest stumbling blocks would be Crimea and the LDNR. I think there the best way forward would be to even federalize Ukraine completely, but no autonomous People's Militias, and two state languages for the country, and a recognition of Russian Crimea. It would be the selling point to have Russia essentially pull the plug on the rebels. With a fully federalized and neutral Ukraine, this could be sold. This would be a bitter pill for Ukraine's ruling elites, but given the scope and scale of the issues at hand, they're the last people I'm worried about.
EDIT: I think the west needs to come up with a diplomatic solution to this other then "Russia you better stop". That one obviously doesn't work. Something needs to be given, more then assurances. And a new framework needs to be created. Russia has put their ideas/desires on the table, and they don't look good. There needs to be a counter-proposal that isn't simply "we can talk about weapons control, but you can bugger off on the rest".