1) I like your set up
2) If I look at your dates, I wonder wether 500 AD really makes sense. REal Action just starts at ~750 Ad which would be a fair compromise, wouldn't it?
3) Netherlands: Wikipedia (yeah) says the HRE lost influence in the 11 and 12th century, which also corresponds with the rise of these trading cities there (Antwerpen, wool, etc. ...) if I recall correctly. They then get conquered by Burgundy. Perfectly fine to start them around that time 1050 as for example "Flanders" which goes in the same direction of generalization as before seen.
4) Burgundy really needs an interesting part of transition: Being founded 870 as part and then central and surviving part of the middle empire, around 1000 AD it gets (finally) corporated into the HRE (= vassal status), loses more and more territorry and then reemerges in different fashion as the house of Burgundy with a bit different territorial basis (including the lowlands). Though worthwile!
5)If we have RFC, we can see Novgorod as a predecessor of Moscovy... I just saw that we need to make the Norse really strong if they should first be a larg player in the West (Britain, Normandy, Raids) and then or at same time expand also to Russia (and Sicily and constantinople) and afterwards into Germany and Poland... Perhaps a separation would make sense nevertheless?
6) Austria: first date! Switzerland 1290 &1650 are perfectly fine Hungary: Please no Avars...
"Arabs": Agreed -> Question of Maghreb (esp. Fatimids?) Venetia & Genoa: Agreed, though Venetia could start a bit later; Milan/Firenze (aka Lombard League) about same time as Genoa.
7)@say 1988 Papal States were a state like any other, it fought wars, suffered Saracenic attackers, grabbed land et al... Canossa is a good example or lather Julius II. Read a bit about that "pope". The Sacco di Roma was not for nothing... I say, make them playable... ; Mongols: They were too little time on the stage of European history, although they had a huge impact they can be represented by Barbs (culturally
) same goes with Timurs/Tamerlanes hordes. ; Ottomans are sufficient for our scenario, Seljuk turks would be the same, and we already have Turks with the Mamluckes.