Some think Russia's objective is to occupy/annex everything East from the Dnepr river.
Jacques Baud in his book (The Russian Art of War: How the West Led Ukraine to Defeat here >>54630) writes this:
In 2014, many voices were raised, in Russia, calling for intervention, as the new regime in Kiev engaged its army against the civilian population of the five autonomist oblasts (Odessa, Dnepropetrovsk, Kharkov, Lugansk and Donetsk)
I highlighted these plus the currently occupied areas (Crimea is not really occupied but de facto Russia, but some would say it is).
They might want those oblasts where they can count on most of the population, ie the "autonomists". From a legality point of view they can say they try to protect the Russian natives from the Ukrainian govt. They also might want to add Mykolaev to round it out.
If they can collapse the Ukrainian army, or the state itself (perhaps the regime could be enough) then they might just take what they see rational.