Let’s assume that President-elect Trump is serious about wanting to wind down the proxy war in the Ukraine. Of course, the US cannot stop the Ukrainians from fighting – all the new President could do would be:
(1) to promote actual democratic elections in the Ukraine,
(2) to cut all support to the Ukraine until such elections take place, and
(3) to try to assure Russia that NATO is not a threat to them.
Hopefully, this could help persuade the newly-elected Ukrainian government and Russia to jaw-jaw instead of war-war.
Point (3) is the tough one. One approach would be to insist that NATO’s article V promise still stands, committing support to any NATO member who is attacked. But as a demonstration of NATO’s [new] non-belligerence, US/NATO would remove all foreign forces from all of the countries which border Russia; would remove all pre-positioned supplies and missiles from their territories; and would cease all NATO joint training in the territories of those border states. The border countries would of course still maintain their own indigenous militaries.
The question is whether it would be more effective for the US to make that change to NATO pre-emptively as a demonstration of good faith, or to offer it as a bargaining chip provided Russia agrees to negotiate with the new Ukrainian government.