Mysterious Game beginning in 1337 instead of 1444 means so many interesting scenarios and alt history divergence points that we couldn't see in EU4, I a m a big fan of this refreshing date even if it may be difficult for the game's later pacing. Beyond what was said in the OP, 1337 also offers:
- Yuan Mongol dynasty ruling over China, entangled in wars with other Mongol empires, Vietnam, Korea, Japan and most importantly itself; so many ways it can go
- Japan being threatened by Mongols and in a completely different political era (shogunate not yet established)
- In SEA Khmer and Majapahit aren't yet in the phase of rapid decline (also Sukhothai instead of Ayutthaia)
- The entire Hindu India being threatened by the enormous Delhi Sultanate, which is also ironically its shield against Mongol invasions
- Mamluks being fully operational
- Very different situation in Iran and Central Asia (and imminent Tamerlane)
- Golden Horde changing the entire Eastern European dynamics
- Anatolia full of small Turkic states
- Ottoman travel from zero to hero (?)
- Thriving Balkans with numerous countries (including playable Bulgaria) which are not masochistic to play yet
- Numerous Rus principalities struggling against Golden Horde
- Pagan expansionist Lithuania which can go in many unpredictable directions
- Extremely different starting situation of Poland (struggling against many similarly strong threats on all sides)
- Playable Ukraine
- Very different political setup of HRE
- Hussite Wars
- Mighty Hanseatic League
So many interesting and new scenarios!