They've posted reasons as to why each faction is opposing nagash. They're actually fairly decent. Except one. Apparently the Slann do it cuz we have a freaking oath to sigmar. Since when do slann do oaths, let alone oaths to anything that isn't the old ones?
I think the issue is going to be that they still have this "great unknown plan". the way it's going, the only way to further the Seraphon lore is if they get a dedicated expansion based around some trigger to their plan kicking off proper...mush like the death reboot really. Basically they are written into a corner of being 100% mystery or 100% main plot driver.