I'd argue for a very late start date, around 1300.
I always like the latest start date for a shorter, more focused game. It's also a time of much stronger, more centralized and established kingdoms, making a Vassal Playtrough (or at least starting as one) very interesting and much more challenging.
Especially since CK3 seems to expands quite a lot on roleplaying, making playing a vassal probably a lot more interesting. Trying to expand your powerbase while under a strong King I imagine could be quite a lot of fun.
I also like that at these late start dates the foundation for the modern era are already set, with a Christian Iberia, falling Byzantine Empire and Ottomans, Russians pushing back the Mongols, the Pope gaining in worldly influence, England having the Magna Carta, and so much more.