Persia and Ottomans for me.
I wouldnt say it was a mistake. But it certainly should come soon with one of the first DLC.
Although the number of civs that are always present in Vanilla is also shrinking (14 for I and II, 13 for III (no Mongolia), 11 for IV and V (no Zulus and Babylon), 10 for VI (no Aztecs)).It's totally their call which civs to include, although we all understand there are certain ones that will always be there (America, India, China, Russia, Germany, England, France, Greece, Rome, Egypt).
Although I miss Persia, I'm excited about the possibility of that "300 scenario" that GauntZero mentioned.
It should include Persia, Sparta and I hope Babilons, Phoenicians and other ancient civs/leadears.
I`m surpriced the Netherlands is not in it. Would be acceptable if it takes its rightfull spot with a cool trading/economic mechanic. + Unique naval unit.
This little nation dominated in its golden age. Still pushing far above its weight in present.
Certainly not before Persia or the Mongols though, right?