Residents of Estonia by ethnicity.
Belarusians 1.1% Finns 0.8%
Up against the Russian border, Ida-Viru county in Estonia is about 73% ethnic Russian. The other 14 counties in Estonia are largely Estonian.
Putin could probably walk in and take Ida-Viru but that might be his last play. No other region in Eastern Europe outside of Russia has a majority ethnic Russians.
Any other region besides Ida-Viru in Estonia(73% Russian) and Crimea in Ukraine(57% Russian) would be an entirely different circumstance.