Long-Term Infiltration

By the incentivizing of Russians to settle in the Donbas region, and exporting or killing millions of Ukrainians, the proportion of Russians to Ukrainians has changed appreciably in favor of the Russians, who now make up about 17% of the population, and constitute a large majority in the far eastern provinces and Crimea.

From a tactical point of view, a pro-Russian population would be more likely to support troops on the ground. They would also be a natural resource for recruiting and operating an intelligence network. Additionally, they could provide a handy legitimation for invasion.

Russia is technically opposed to any nation being involved in the hostile takeover of another nation. As a permanent member of the UN Security Council, Russia is tasked with policing the conduct of world nations. As a result, Putin is constrained to provide some show of legality for an aggressive move on Ukraine, and his main argument is that Russia is defending democracy and self-determination of Russian neighboring states. He bases this on transmogrifying Russian-heavy Eastern Ukrainian provinces into independent nation-states.

Yeah, it does sound unlikely.