I always wanted to see spiral staircases, that your Sim can slide down and it puts them into a playful mood. I also think it would be cool to report your neighbors to the Police for noise in the City Living pack, noise complaints could then be involved in the action play career! I'd also love for Sims to complete a first dance at their wedding as part of the goals, I think that would be really nice.
Some bigger ideas I have (but no idea where to post them...sorry I'm new to the forum):
Action play Fire Fighter career
Burglars & raccoons make a come back like in the original Sims
Being able to hire a Photographer for events