As much as they would like to, Turkey has no ability to expand in the north/west. In the east. Georgia and Azerbaijan would probably ally with Turkey, and Armenia would be easy pickings. Maybe Russia's Caucuses declare independence or even side with Turkey depending on which way the wind blows, but that's about as far as it could go. Beyond that, Russia would probably use nukes to defend their sovereign territory. Syria would fall, and Europe would get the pipelines from the Middle East that Russia has been blocking.
But Russia still has huge oil and gas reserves. A "declined" Russia just looks like a more corrupt Saudi Arabia. Now if the Eastern republics free themselves, say east of the Urals, then Muscovy becomes North Korea. But that only happens if China backs them, and at that point, the old saying "why buy the cow when the milk is free" applies. China will already control Russia.
Have always thought that China would take the opportunity to expand into eastern Russia, but I don’t see China being in such great shape as many say.
There are lots of internal issues with their economy that are being hushed, real estate market for one, manufacturing moving out another, and I think they have a looming food issue coming
Time will tell