I also wouldn't mind seeing allies at least offer to come to the aid of attacked allied provinces, or alternately threaten an attacker with sanctions (sort of like the Pope does if a Catholic faction attacks another).
Perhaps the strength of an allies reaction could be linked to the time of the alliance - ie the longer standing the alliance, the "heavier" the response. For example, Italy attacks Sicily, who in turn has Almohads (10 years) and HRE (2 years) as allies. Next turn emissary from Almohads advises that they declare war on Italy straight away. HRE emissary advises that if all troops do not withdraw within 2 years....yada yada.... stay away for 10 years, then they will declare war on Italy....
That would add another dimension - because suddenly you have to be mindful of the possibility of finding yourself at war with several factions on a number of fronts
It also would replace that silly (seemingly random) series of alliance cancellations that happens every time another faction attacks/player attacks another faction. Say Italy has England (10 years) and Spain (3 years) as allies. England would also declare war on the enemy nations immediately, Spain would issue a warning.
Another thought as an intermediary step....at present you have enemy, neutral, and ally as the 3 possible statuses of other factions....wouldn't it be good if "neutral" could be split into say "neutral but happy to trade with you" and "neutral but imposing trade sanctions" in which case trade was restricted (say halved)...
Like I said, just thinking out loud....
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