In diplomacy, could we have the option to ask someone to declare war on someone else?
With basic checks for "high enough relationship with me" and "low enough relationship with the target", a comparison between the potential ally's and the target's military might (possibly also population/industrial capacity/tech ?), an Influence cost and maybe a money or metal cost as well (perhaps if relationships aren't that friendly but not under the "help" threshold).
And if you worry about this being possibly abused too much, also a condition that the player must already be at war with the target. Or a softer limit, with highly increased costs if the player is not at war with the target, and maybe even a discount if the player is the one being attacked (or if the target is much weaker).
It's annoying that the AI often asks me to attack someone else (who they might not even be at war with yet), without offering any incentive, and the time I'm being invaded, I don't even have the chance to ask for help.