As have been said many times on other threads, expanding the timeline back to the 600s or even the time of Charlemagne would be impossible in terms of research. Yes, there are quite a number of shortcuts currently for research, but even then there's still some historical plausibility to them. Before the 900s at earliest, you're looking at least, literally three quarters of titles with rulers who cannot be figured out from normal research - and some might say PI can just have made-up rulers. Fine, but then there's the problem of titles: who gets what titles, who owns what provinces, and not to mention that a large portion of the titles from the current timeframe wouldn't even exist several hundred years previously, meaning somehow not only does PI have to do research for new titles and de jure boundaries, they have to figure out a way to have them change over the course of the game.
This is something that cannot be done with the current CKII. At best it could be a standalone expansion with an entirely new de jure map, titles, among other things, but there's no way the game in its current form can handle anything before the early to mid-900s, and ideally anything before c. 1000.
As for dates past 1453, I think going up to 1492 (i.e. the year of Columbus and the finishing of the REconquista) would be reasonable. The discovery of the Americas *might* be handled by events and decisions (now that we have the Aztec DLC and all), wherein the timeline could be pushed up to c. 1530 at best, but nothing past the Reformatino, which would require EU-style mechanics.