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).