In my opinion, the Editor team is putting the cart before the horse by working on the UI details before figuring out how this content is going to be stored.

Gregory Schoppe put together a good article on this topic, but from following the conversation in Slack, people were more interested in discussing the nitty-gritty details of browser support than address this fundamental issue.

I currently use ACF's Flexible Content field to build all sorts of layouts, from slideshows to tab boxes to logo lists, and so on. I'd like to see something that could replace that built into Core. Right now it looks like we're just replacing what the tinyMCE editor can do. But I don't know everything, so maybe this is just phase one.

What I would like to see is a system that developers can hook in to and register custom blocks, much like how we register Custom Post Types today. Each custom block would have its own fields, or even sub-blocks (e.g., like a Slideshow block that has sub-blocks for individual Slides).

As a developer, I like to get these sort of structural questions figured out first, before heading in to design. If we overlook opportunities to scale now, then we'll end up with something that isn't very flexible for the future.