My opinion is that Putin took Crimea and started the war in Ukraine because he wanted to test how far NATO would go. He can see that he can pretty much get away with occupying territory as long as it belongs to a weak state with the only repercussions being a few sanctions. As for attacking a NATO member, I'm not sure if he would go that far. Maybe if another event diverted the world's attention but that would have to be pretty huge.
Crimea is strategic interest for Russia. It isn't about testing NATO. The opposite was occurring prior to Ukraine flareup. NATO was testing Russia, expanding missile systems(Defensive and Strike capabilities) around it.
I doubt it very much, this is a border/domestic war which probably has been brewing for many years. If there is another World War it will be more likely to be the Middle East where it becomes international. They have come close to resolutions several times already which is a good sign, as negotiations always take a long time.
I can't see how a world war would take place in the ME. If more nations breakdown, and there are resources at stake. Nations may compete over them. It won't be a nuclear war though. The Middle East isn't capable of projecting power. Only thing that is capable of is bringing world down with it due to one factor: instability. I know some Christians and Jews believe war of 'Armageddon' will take place there. That is unlikely without foreign support.
It will only occur if Turkey becomes strategic ally to Russia. And Turkey opens its waters to Russia. Together along with other elements they would have to take down Israel. For strategic purposes. I still don't believe it will go nuclear. If such scenario occurred.