As far as I can tell, the ERE is the only christian empire existing at the 867 start date. Kingdom of France is localized as Kingdom of West Francia, while the Kingdom of Germany has been localized as the Kingdom of East Francia.
The isue is that, while Charlemagne's imperial title was still knocking around, it didn't function like an imperial tier title would in game. However, I wouldn't be suprised if they did something like the petty kingdoms for the imperial title.
Not sure about the specifics, but I'm sure if nothing else you'll be able to form it once you've gained control of 80% of it's dejure land.
According trailer there is "settle down" decision for nomads so i hope we can expect special mechanics for nomadic tribes. That's great!