So, as long as NATO would agree not to put nuclear weapons in Ukraine, that would be the same situation we tolerated for decades with Cuba. Military forces okay, nuclear forces not.
Of course, given that Russian ballistic missile submarines already can legally get closer to New York, Washington, L.A., etc. than Ukraine is to Moscow, I'm not sure they'd have grounds for complaining anyway.
The term NATO expansion isn’t quite correct, more like NATO accepting new members.