He would need support of local population for that. And if locals want it, why not? Right for self-determination, after all. At least that was the excuse when USSR/Yugoslavia/etc were broken down.
But it is not that simple; any such territory would be a drain on Russian economy. For now, it is tactical advantage to keep Europe and US to drain theirs here. (One of reasons, why opolchentsi stopped before Mariupol few years ago).
Ukraine is has fantastically fertile crop land, and Russia is today a net importer of food. Food has never been a trivial concern for Russia or Russians.
