Please add a bonus like in some of the older total wars for when you unite your entire faction (through confederation or otherwise) and a cutscene. e.g give the empire a faction wide leadership buff and +3 lord and hero recruit ranks on top of the 'pre-existing summon the elector counts' button. Or a super powerful unit summon for lizardmen. On top of this, make it possible for most factions to revive dead factions, much in the way that the empire can restore dead elector states.
Also please tweak confederation in the diplomacy menu to make it easier for factions that don't have any dilemmas or special mechanics that facilitate confederation (e.g orks, Norsca etc). For chaos, undead factions, skaven etc this unification event would also trigger the end times, making most factions your enemy, whilst giving you massive military and faction buffs. e.g tomb kings and vamp counts re-awaken Nagash and all unite into a single faction, or become permanent allies.