Thanks for sharing all your thoughts, I know how time-consuming it can be (especially when you’re not a native English speaker).
I agree with many of your ideas which are in line with my own suggestions (a bit outdated, I’ll remake that civilisations list), especially regarding Eastern Europe. In AoE II HD, half of it was compressed into one single “Slavs” civ, how unfair that was. Magyars had their own civ but it was quite badly conceived and nonsensic.
Wallachians/Vlachs, Kievan Rus and Poles definitely need their own civilisations.
I disagree on some points. First, I think civilisations designing and naming should stay within the AoE II spirit, in which civilisations portrayed cultures rather than states/dynasties, hence them being named with ethnonyms or demonyms only. It is less clunky regarding timeframe (what is the Delhi Sultanate before the 13th century…?) and gives a feeling of freedom: your choose a culture and you create your own state.
While I also suggested to include the German Knightorder/Teutonic Order as a full civ, I don’t think the Holy Roman Empire would make a good civ for many reasons, mainly its lack of cultural unity. Teutons (i.e. German(ic) speakers of the empire) would already be more relevant, though I would still split them into at least two civs: Low Germans and High Germans (those names are based upon linguistics and I still have to find proper ones). Here I explain my views on Teutons/HRE.
Instead of “Seljuk Dynasty/Empire”, I’d add Oghuz/Ghuzz, the tribal confederacy from which emerged both the Seljuks and the Ottomans and thus perfect to portray those Western Turks.
Abbasid Caliphate is one of the few dynastic states that lasted enough long to be seen as a proper culture so I mostly agree. I would just rename it Abbasids so as to stay within the AoE II frame. Here my views on Arab civilisations.