I've been thinking about this the last few days.
Imagine you're a (neutral) negotiator between the Ukraine and Russia... how would you solve this problem/war?
Russia says it wants to re-establish Russia a world power, just like it was in the times of the USSR. To be honest, I think he's mostly after the land, resources and food that the Ukraine provides.
He also says he doesn't want the EU and the NATO at Russia's doorstep.
Ukraine - well, they just want the war to be over and the Russians to leave. They also want to Westernize, and minimize the influence Russia.
I think the best way is when the EU and the US threaten Putin with a massive war he can't possibly win and in return give Russia some favorable Trade Agreements that Putin can wave around saying he bargained for a great price on food, oil, gas etc.
To answer the OP, I think the best potential for a negotiated peace* rests on:
a) Ukraine agreeing to permanent neutrality in political, military, and economic spheres.
b) Permanent acceptance of pre-2014 Russian military bases in Crimea.
c) Formal recognition of the Russian language as co-equal with Ukrainian.
d) Some sort of 'de-nazification' for Russian tv cameras.
e) Various safeguards to ensure democratic representation of territories under Russian military control.
C is far and away the easiest. Formal recognition of a language is hardly unusual, and frankly I feel is something Ukraine should have done a long time ago.
D is easy but politically unpalatable for Ukraine; though I'm sure if push came to shove the Ukrainian government could dig up some right wing nutters (like the Azov group who even the US government expressed unease with) who aren't thrilled about having a jewish president for some prison time.
B is also probably doable, as unpalatable as it is.
E is also doable, but going to be tricky in the details as nobody trusts each other. This would be stuff like no mass expulsions of those considered to be 'disloyal', full voting rights, etc. Its going to be long and messy, requiring something like the stabilization and trust building missions that are still running in the Balkans.
A is the single biggest problem, and unfortunately, probably intractable. During the Cold War Austria and Finland could get away with permanent neutrality, but I don't think the political or economic conditions today fit that. Any sort of economic treaty with the EU is going to be viewed in Russia as inherently political. Any attempt by Europe/America to push Ukraine into accepting permanent neutrality is going to be viewed as a betrayal.
It is clear the "fighting Russia to the last Ukrainian" isn't happening; and Russia is, you know, the invader. Given how A is basically intractable at the stage, I don't see the potential for a negotiated peace any time soon.
If I was in charge of US foreign policy, I think the best approach is to continue and step up military support for Ukraine, while also making it clear to them behind the scenes to either knock it off with the 'freedom fighter' attacks inside Russia, or at least do so in a way no American equipment is used at all.
*ie Ukraine hasn't established control over its 2014 borders and the Russian military hasn't been completely degraded.