It should be remembered that the eastern Ukraine, including the Donbas, is populated with a majority of Russians. I imagine Putin would be looking to annex all of the Ukraine east of the Dneiper-unless he wants the whole thing.
One thought is that Ukraine was be Federated, much like the former Czechoslovakia, with more or less two Federated areas with a weak central government in Kiev.
I was just looking at a map of the Russian ethnic majorities. Actually, they are the far eastern provinces, but also those immediately north of Crimea and over to Moldova.
I understand there are Russian troops in Moldova, but I cannot recall where I heard that...so I could be wrong.
If I were Putin I would cut off the Black sea and land lock Ukraine. Then the oil and gas pipelines from the Middle east would be controlled by Russia.
That would pretty much put Ukraine back to third world status at the whim of the Russians.