A review of a book that discusses this and raises this as an issue.
http://blogs.lse.ac.uk/brexit/2018/07/03/book-review-collapse-europe-after-the-european-union/
I'm reading at the moment Flash Point by the eminent geostrategist George Friedman, who thinks it is not a matter of; if the EU will collapse, but when and Europe's history for the last 500 years has been one of many European nations at war with each other in its bloody history which has largely but not entirely been on pause since 1945.
Exactly right, and if the EU collapses War is the very real possibility. That is why Winston Churchill famously stated after WW2:
"We cannot aim at anything less than the Union of Europe as a whole, and we look forward with confidence to the day when that Union will be achieve" Zurich speech September 19, 1946
He stated this believing Great Britain would take the lead in Europe and together with the other States of Europe would work together to reduce the risk of conflict to the very minimum.
But as you state, with a vacuum being created in the power balance of Europe, a strong aggressive power (like Russia?) could step in as it takes advantage of the weakened situation, as Hitler recognised the weaknesses in the late 1930's of the major powers to be willing to "get involved" again in conflict after the dreadful costs of WWI. He took full advantage of this during 1938 and 1939 and then went forward from there boosted by the pitiful response from Great Britain and France, who were at that time at their weakest. Putin, or someone with similar visions, could well do the same.
As for International organisations, like the UN or NATO, taking decisive action to stop any deterioration towards war by issuing threats, forget it. International law is toothless; always has been, always will be, as it relies on the strength of usually just one supreme power, which currently is the USA. With the current political landscape in the USA being one of 'America First', and the incumbent President reviewing the costs of continually propping up the security of Europe, all we can expect in the event of war in Europe is a situation, as GB and Churchill witnessed in 1940; a isolationist USA who will not risk millions of young American lives and incur huge costs to run to the defence of a pitiful Europe that has done nothing to increase it's military spending to the level expected.
So, in short the EU
will implode, as all 'empires' do, and all I hope is that Putin's Russia implodes before then.
On that subject, Russia, I will again quote Churchill:
"I cannot forecast to you the action of Russia. It is a riddle wrapped in a mystery inside an enigma." Radio broadcast 1st October 1939
Especially after witnessing Russia hosting the World Cup, and it's great PR achievement to hide the real, mixed up, truth of Putin's country, doesn't that quote still ring so true.