Not feeling the difficulty level thing... or wanting to turn ts4 into ts2. (Not you but simmers in general) The only thing I like about ts2 is the detailed animations. Ts4 feels way more sandbox in a good way and I don't want difficulty to equal limitation or uncontrollable circumstances like story progression.
But I'm definitely up for more lot challenges, lot traits, burgerlers, natural disasters (can also be a lot challenge) the kraken, moon phase zombies,
Stronger negative emotions.
Traits could add a bit more challenging impact too. For example I created a scary looking vampire and no one cares. So I downloaded a custom "scary" trait and now when my scary vamp goes zipping by sims get scared? some of them start panicking or feel uncomfortable, so it makes it a challenge to go around in life as a scary vampire lol.
More vampire break ins. Would be nice to control how often (still never get them. Just a vampire standing at the door)
I also think the paranormal pack is a great example of a good challenge.
One sided love would be nice as well as a more expansive sickness system where sims can care for sick sims, vs having to by medicine for myself and cure myself of the illness I never even get anyway.