Based on conversations on another recent thread, I've actually been considering something similar to this, but not quite the same:
Separate from the military diplomatic relations, have a set of cultural diplomatic relations. Something like
Brothers - Friends - Indifferent - Hostile - Enemies
They would have no mechanical military consequences (though they might have other mechanical consequences, at least in time), but would be a way to clearly indicate how your realm and another realm feel about each other.
To be honest, I'm not thrilled about the idea of allowing any kind of attacks on a realm without being at war with them. It complicates things, particularly when I've just introduced a mechanic requiring reasons for being able to attack a realm—which depends on having to declare war to attack a realm. Furthermore, I think it highly likely that 99% of the time, "hostile" would be used and seen as nearly synonymous with "war"—certainly, a realm having its regions attacked and looted isn't going to hesitate to treat that as an act of war.
And to be clear, you can already attack a realm you're neutral with by setting your men to Aggressive or Murderous. You can even loot them.
