They're never going to add full actual religion into the game, and that's okay. But what if they did

I think one thing they could do is create a unique religion to the Sims, that borrows elements from all kinds of religions, and your Sims have a choice to believe or not, and you can of course create your own as well, using Seasons ans GT, but there is a more 'official' one, that creates a trait, major holiday and maaaaaybe an aspiration. Even make it about belief in YOU, the player, as their deity with the belief they are controlled by a green diamond and the physical manifestation of you is the Freezer Bunny.