I think having things like cancer in this game is way too dark and would probably bump the rating up. These kinds of things are what mods are for if you're that determined to have it in your game but I don't see EA making a adjustment to the game like this.
If we tweak the idea a bit though, having things like your sim being colour blind wouldn't be such a bad idea.
Personally I think I mentioned before adding some birth marks or skin shading to the game (so people with skin conditions like vitiligo could make themselves in the sims without mods).
But that's about as far as it should go.
No super dark real life issues like cancer and stuff like that should be in the sims.