A slip dress is one garment and at least four outfits. The difference is entirely in what goes with it.
A fitted cotton tee underneath turns an evening dress into a daytime one, and solves the shoulder question at the same time.
An oversized blazer over a slip is the easiest smart-casual outfit there is. Keep the dress plain and let the blazer do the structure.
A belt at the natural waist gives a bias-cut slip a defined shape without altering it.
For evenings — with the right lining and a fabric that has some weight, a slip needs nothing added.