I think they could add religion, but only if they gave it a different euphemism (like how wine-making was called nectar-making).
As long as they kept it funny and light-hearted, it could work out okay.
Personally, I would like to see them add things like diseases, addictions or disabilities. Can you imagine having a Sim in a wheelchair?? That would be amazing. It would have a huge effect on the game.
I think if the game could simulate more of the real life challenges people have in their real lives, it would be an incredible experience.
Unfortunately, the design philosophy for the Sims has always been quite gentle and light-hearted, so I doubt they would ever add things like that. But if they make The Sims 4 and they really want to take the game to the next level, they might have to consider it.