I'd like a romance gp. You could include a full dating system with turn ons/offs (or some sort of preferences to differentiate sims) and online dating/via the phone. Slow dance would be nice, more wedding gameplay. Also new lingerie in CAS - I can't remember the last time we got any! There's lots of little things you could do, the main feature being a preferences + dating system. It would be similar to parenthood in that way, with the new system being the feature and no world.
Also you could release it around valentines if you really want to be on trend.