A China mechanic is probably the biggest. Hopefully the new game/engine means they won't have any reason do that again should they add Asia in the future.
Societies are a love/hate thing for me. They're usually fun to be in, but I also feel compelled to join one because of the bonuses you get; not doing so feels almost like deliberately not choosing a focus. Yet not every character/religion/region combo would have a society that fits them (especially if you tended to avoid joining any Satanists... not that I hate supernatural/fantastic events- quite the opposite- but I never really got into their mechanics, at least not untill after they were nerfed to the ground) leaving you feel like you're playing the "wrong" way. The idea on the first page or two of having custom societies that start between just some friends sounds a lot better... especially if they could transform into the silly map-spanning order or conspiracy that societies already start at no matter how implausible that is.
Finally, I hope Islam is much better than it is in CKII. Playing a Muslim character always felt dull and bare-bones compared to almost any other religion (only non-heresy that felt less fleshed out for me was Jusdisam, and yet even then there was at least the challenge that you were the only Jewish ruler in a region with your neighbors happy to Holy War you)