The endgame is first buffer zones against the West around Russia, and if possible, restoration of the pre-USSR Russian Empire. That's what Putin says, and it's consistent with his actions.
First South Ossetia, then Crimea, then Belarus, now Donetsk.
I don't get why Russia wants a buffer zone. Russia is huge and has proven time and time again that you can invade but you are just going to be destroyed.
Access to the Caspian sea, sure. Oilfields, sure. But the one thing Russia doesn't need is more land.
Historically, food. Ukraine has a good climate for agriculture and large flat areas with water, like the American midwest. Most of Russia is marginal for agriculture.
First South Ossetia, then Crimea, then Belarus, now Donetsk.