Thanks for the welcome. I hope I didn't put my foot in with my first post. There is too much information on each page to take in fully at a glance.
I think, on purely economic grounds, a Northern alliance will most likely follow an EU breakup. Most countries bordering Germany depend on the German market. This will create a bloc other countries will join by and by. Any Southern alliance is likely to be dysfunctional.
The big question is France, the French economy is very closely connected to the German economy. Yet, joining the German bloc as a junior partner is going to be very unpalatable to the French.
Border issues will arise in a later step if conflicts over economic issues escalate. There is after all a chance that any new alliance will incorporate many of the achievements of the EU and that thus the worst can be avoided.
Anyways, it seems that the South will probably be the big looser in any new arrangement. And France will face some very difficult decisions.
Incidentally, my mental model is more 1914, which is suiting since we are soon to celebrate its 100th anniversary. There is a new game of alliances, unthinkable a few years ago. The Berlin/Paris axis appears to be somewhat shaky and there seem to be a number of think tanks which try to put a positive spin on a new London/Berlin axis.