Pink, scarlet, and purplish-gray clouds to add to skys for sunsets/rises? Or more clouds in general?
Solid-colored or gradient backgrounds would be interesting too, especially when paired with separate ground objects to construct your own backgrounds. For instance combining the clover field, a solid sky blue, and clouds to create your own environment rather than relying on single already-complete background items. Of course some combination is already possible, but I'd really like items that are made purposefully for combining rather than being coherent on their own, since combination is popular among dressup-ers.