We would like to be able to embed dashboards, apps, or websites into Slideshow because it gives us a space where we can put in a small amount of text without having to add the bulky Sidecar. Having the text either hovering over the content allows the users focus to be on the content and not going back and forth between text and content. Sometimes a particular section or interest is only a sentence or two and we don't want to be confined to either embedding something in a narrative block or in Sidecar.
Plus the Slideshow breaks up the monotony of constant scrolling when you have longer story maps. It has been brought up several times from a lot of users but it can't be stressed enough how important it is to have some sort of table of contents or quick way to jump to different sections in a story map (à la Journal). I love Slideshow but just think the added benefit of embedding would be a big addition.