Since the Empire AI has been struggling to make it to the end of the game without being crushed, being basically surrounded by enemies on all sides, it’ll now start without the separatist threat, owning the full Reikland province.
This should give them a much needed boost, but applies only if the Empire is AI.
The one exception is the Norsca Race Pack (Wulfrik the Wanderer and Throgg the Troll King).
These have yet to be implemented and will become playable in a future update.
Including all the core Races and Legendary Lords from both games, Mortal Empires also allows you to begin a campaign as any DLC or Free-LC Legendary Lord from the first game.