I feel like Ideology fulfilled something that was missing from the base game, that being religion (and slavery too I guess). If I had to point out something else that was missing, it would be pregnancy. There's a good reason why it isn't in the game already. Because it would require so many new mechanics: genetic makeup, teaching, maturity stages. Mods like "Babies and Children" already have this covered, but I can still see this having an official spin, similar to how we had slavery mods before slavery was officially added to the game.