Yeah De Jure Empires seem strange to me in a sense. With the exception of the HRE, Persia and Byzantine Empire, there wasn't really enough history for other Empire's to warrant de jure territory.

I doubt it's really possible outside of DLC, but I'd love if they added an Empire mechanic, with some sort of Imperial Authority - low imperial authority means low tax income, less troops from vassals, and higher vassal autonomy (Declaring wars amongst themselves, against foreigners, etc), then if it reaches 0, the Empire collapses.

Succession would then be an obvious thing that crap's all over your authority, depending on the heir. So a baby heir, 0 in all skills? Bye bye Empire unless something short of a miracle occurs. But a 26 year old strong genius? The empire continues!

Then maybe tie in laws that take time to enact, that can counter some of the negatives and stabilise your dynasty's authority, thus over a century or two, you should be able to end up with something akin to the Byzantine Empire, that won't collapse on succession, but might be more prone to civil war and wars against tyranny.

I just feel like Empire's need more flavour, and need to be something that feels somewhat unique to play. It shouldn't just be a Kingdom on steroids - in fact, I think Empire's are easier to manage simply because you can give out Kingdom titles, essentially turning your Kingdom in to a really big fucking Duchy. Like a Duke you've only got something like 6 vassals, but each vassal has like 8k troops instead of the 800 count vassals have for Duke's.