So many times I wish I could add instructional text, video or images in a CMS collection to help my client understand certain fields better or maybe describe in detail on how to get the best out of any content they will upload for dynamic content. 

I find that the help text a lot of times is not enough. It helps but I feel a rich text element dedicated for this would be much more beneficial in a lot of use cases.

I feel like this would make things easy for designers and developers. If you think about it, Webflow is made for designers really and when designers deliver content it's common to give brand guidelines to help clients understand how to use it as best as possible so everything stays looking great and consistent.