My immediate reaction when hearing that "Crimea used to be part of Russia until 1954, you know" was that Russia should give back Karelia to Finland.
Yeah, me too. These kinds of politics, well, they cannot be honest, if you think about it. How many good, honest reasons can you think of, for one country to roll into another, and start uprooting the place?
So it's obvious any justification is gonna be a bunch of hypocritical bullshit.
For Russia to make a move on the EU or Nato would be incredibly unwise, so that would be an enormous surprise. For geopolitical analysts, their invasion of Crimea is not that huge a surprise, it is a spot waiting to flare up, like Abkhazia and Ossetia was.
Another possible flashpoint can be, for example Belarus, who has "promised" a unification with Russia. If Belarus were to back out of this, and seek closer ties to Europe, one could see similar events unfolding there, but Crimea stands out because it wasn't always a part of the Ukrainian SSR (or ever), Crimea has a long history of being it's own Soviet Republic. This sets some precedents, people remember, and for example, Caucasus used to enjoy its own autonomy as a separate Soviet Republic for a short while. They have never forgotten this. As soon as the Soviet Union broke down, the Caucasus republics, most notably Chechnya, wanted to join all the other new nations into independence. They even achieved independence during Yeltsins useless rule, and for a while the "Ichkerian Republic of Chechnya" existed as a de facto independent nation, untill - - - Putin, rolled his army into Grozny, and litterally levelled the city to gravel, in one of the most extensive city destructions in post WW2 times.
If you ask a Russian, Crimea "Is Russian" and Ukrainian ownership is nothing but a lucky fluke for the Ukrainians. If you ask a Ukrainian, theyll tell you Crimea is Ukrainian, and ask a Crimean, they'll refer to the independence Crimea has often had in the past - and tell you Crimea is a country of it's own (It has been autonomous under Ukraine since the fall of Soviet)