There are a lot of good ideas for specific challenges here :) These would be fun to see. Especially for things like vampire visits that got nerfed, "un-nerfing" them could easily be a lot challenge.
I'm still more or less with @bixters , I want enough things to be more challenging (or to have the option) that it would be a LOT of lot challenges. But maybe this is a minority opinion.
In Sims 4 I re-created my favorite family from Sims 2, who started out as an immigrant couple with starter money, 5 children and another on the way. In Sims 2 the gameplay with this family was full of angry yelling teen girls because they had to do their homework and didn't have anything fun in the house besides pillow fights with each other, while I was trying to keep the mother from dying during her pregnancy, juggle 3 part-time jobs for the teen girls, and send one of them to the store late at night after her shift so she could buy groceries for her hungry family. It was great. Needless to say, none of that happened in Sims 4. They just had a sweet and happy life and could buy everything they needed after the first few days. I didn't play them for very long. Honestly, I prefer Sims 4 as a game overall, but I wish there was an option to make it harder.