This is more of a "behind the scenes" suggestion, but it could change a lot the way the future campaign works (and mods !)
So win conditions are currently stored in the file that describe the initial state of a campaign. For each faction there is a set of win condition for single, coop and h2h campaign. Most of these win conditions are based on map elements (territories total, specific places to hold, specific factions to be destroyed) and some can be triggered (like the errantry war)
Issue is this is complicated to change (for starter mods that change the same start file are incompatible) and even having triggerable parts of them the UI doesn't change. You could imagine a more complex system behind the scene with a win condition stated as "complete the in campaign conditions" but that would be fairly opaque.
So what if the scripting API allowed to actually update and change (or initialize) the win conditions ? All kind of possibilities would emerge :
you could write those win conditions easily for all kind of new factions without editing the start file.
you could update the win conditions to follow campaign events : if for instance you have mods (or future update) feature end game scenarios different from the chaos invastion, then defeating archaon can be bypassed and replaced by nip Nagash in the bud, or squash the ratmen.
coop and h2h campaign could have personnalized win conditions depending on the player pairs. Just leave for instance the basic win condition for normal situations, but create other ones for HE +HE, Empire +Dwarfs, Lokhir + VCoast.. the sky is the limit.