I have to second this. I'm in the early days of using Adobe XD in my workflow, and was thinking similarly that a feature within Designer or a standalone prototyping app would be an incredible addition to the Affinity Suite. Currently there are some powerful plugins working with XD to enable me to publish functioning prototypes without coding the interactions myself, although I usually go back in to refine and add additional code. The ability to not only wire-frame and prototype in high fidelity but also publish functioning mockups all from within Affinity would be a masterstroke. Goodbye Dreamweaver, XD, etc.!