I agree, especially in light of those allies, or even just neutral factions, sending you messages that beg for help against a conquering enemy. That the only option to secure peace is to declare war and utterly destroy that enemy seems overly limited.

Agree to all the above (oh, hi Louist, been awhile). Additional suggestion: Perhaps creating a 'protectorate' status/relationship?  A relatively weak sov may seek the protection of a stronger sov.  Details?  All external diplomacy is handled by the protector.  Declaring war on protectorate triggers war declaration on protector.   Treat them as one 'domain' when using troop strength / power, etc. re AI calculations.  Protector gets to use protectorate's land/roads as if owned.  Protector gets 10% of all resources gathered by the protectorate.  And yes, if the protectorate is running a deficit, that translates into gilders going from the protector to the protectorate.