I've found that the inevitable European backstabs can be delayed substantially by not taking Dijon, Metz, and Bern. Bern and Metz are magnets for the HRE, and Dijon both for the HRE and Milan. If you reach it Antwerp is another magnet (for Danes, Scots, and sometimes English).

Bordeaux, Rennes, Bruges, and Zaragoza are safe though.

The surest way to get into a war with Milan or the Iberians is to grab Ajaccio.

And while I know that the game doesnt have historical ambitions it's a bit funny that the English almost never backstabs the French (medium campaign setting). That is UNLESS you take Caen, in which case they start landing senseless raiding parties near Bruges and Antwerp:p