Just a humble suggestion:
Could you replace the Eastern Roman Empire(or "Constantinople Empire"), by now only ruling Constaninople and its environs with the Crimean Khanate? I guess my reason would be similar to that of the removal of the Latin Empire back in the 1220 Campaign (Crusades)Mod.
The Crimean Khanate was to bring down the Great Horde with the help of the Nogai Horde and the Astrakhan Khanate in the decades to come.
Also, I suggest making the Astrakhan region(NW of Caspian Sea) a rebel to highlight the hotseat the Great Horde was in at the start of the mod.
Having the Crimean Khanate as a playable-faction rather than a Rebel faction/region could further check the expansion of the Polish-Lithuanian Commonwealth in the south-east.
And perhaps you could begin the mod in Winter-1453 or Spring-1454, the latter the year after the end of the 100 yrs War(finally), leaving France and England at peace but with the Pale of Calais the only English-held possession across the Channel in northern France. Currently the English has too much land in SW-France(i.e. Gascony) & Normandy which at the start of the mod, had already lost much of them to a resurgent France.
The final fall of the English in the War came with the re-capture of Bordeaux in October'1453.
With only Calais in possession, a player of England could still plan an attack on France via Calais as what Henry VIII duly did in the 16th century, but only decades earlier.