I still think the real question is why do we need to think outside this stupid box in the first place
However in the MARE NOSTRVM mod we changed the time keeping system so year 1 was the birth year of Philip II and starting the mod around year 80 on that scale giving a good 650 years of gameplay.
It works well (in theory) but it does give major balancing issues.
Province ownership would not change (much) as they have a defined owner without a time frame however the characters will not show up until their AVC date so you would have to change them all as mentioned above to correspond with the earlier year you intend to use. Maybe you could turn the clock back 200 years and make some sort of clever search and replace function or something