I'm going to stick my neck out and predict that occupying Donetsk and Luhansk is as far as Russia goes. And I'm also going to say, so fucking what if he does. They've effectively been independent for 7 years now - along with Crimea. They are indeed heavily Russian areas of Ukraine to begin with. They should have been returned to Russia in the 1990's instead of now and done so peacefully.
The Ukrainians should agree. They don't need or want those Russians in their country. That's been the source of conflict from the beginning - an ethnic tension that won't go away until they break away. I say let 'em go. Any view of the electoral map of Ukraine shows why they don't need or want those areas. They will vote against Ukrainian people's interest and for closer ties with Putin. Without them, Ukraine is free to move with closer ties to Europe and away from Russian influence. I doubt it will ever join NATO though. That's a pipe dream. But I also think Russia should compensate Ukraine for them. Kinda like the US did in the Treaty of Guadalupe. They should buy those areas from Ukraine. And they should pay for it with free gas for some number of years.
I know what people will say: "Peace in our time!" But Putin is not Adolf Hitler and this is not the Sudetenland. The idea of him invading and occupying Ukraine entirely is a fantasy. He'll lose. Maybe he'll crush their army, but he will not crush Ukraine without totally losing virtually all trade with the West and probably significant loss of support at home for such actions. There will be violence on a scale that he cannot imagine, and maybe not even be able to sustain the losses. He's only got a bit more than 100,000 troops for the entire country. It's not enough. Unleashing enough violence to suppress Ukrainian independence could indeed draw in NATO into the conflict. The world may not put up with basically a genocide of the Ukrainian people. Putin also won't be around forever. His days are already numbered likely. Putin just doesn't command the total loyalty of Russian the way Adolf Hitler did for Germans. And he isn't bent on conquering Europe like Adolf was.
It's not perfect, but it is a solution. Going forward beyond this point would be shear madness. I just don't see him as another mad Adolf. And I sure don't see Ukrainians pushing him out of eastern Ukraine and Crimea either.