Best thing for the Ukraine (and the Baltic states) would be for Trump and Putin to agree on a general non-aggression statement that respects borders except for the possibility of sanctioning the transfer of Crimea to Russia as is already de facto the case. There could be discussions about minor changes to borders of east Ukraine where populations are largely pro-Russian. But this should be very limited as there are significant numbers of Russians in many other places such as Estonia, Latvia, Moldova and further west in Ukraine.
I think Trump has some political capital to spend with Putin with agreements on other points such as Syrian security and reset of sanctions. He could tell Vlad, “hey you know big guy, I like you but you can’t have much more of the Ukraine and you can’t go into eastern Europe. Do we see eye to eye?”
What’s Putin gonna say, no? Like all rebellious teens, he needs limits. Give him some and show him how to hit a rescue club. If we can get the Russians to take up golf, they will have a great place to lash out at stuff without killing millions of people.
Putin/Russia won’t stop. Ever. They are rebuilding the USSR.