It’s the Slate code being complicated. I would prefer modifications to the Editor to be simpler to do, even if it wasn’t with a GUI of some sort like UMG. I tried watching the initial Twitch stream that covered editor extensions and it was extremely hard to follow, simply due to the relatively large amount of API I’d have to dig through and understand before I could get what was happening. Something more streamlined where I can look at SOMETHING (GUI or code) and clearly see, “Okay, so this is my toolbar and this is how I add a new button/drop-down-menu/etc to it, and this is how I bind a function to that…” and making it easier for people to follow along with how to do more complex things like creating dynamic images like a Viewport, and whatnot.
I can understand Blueprints not really being something you can use to edit the editor interface (seeing as how Blueprints is, itself, something that is displayed from within the interface and relies on internally generated data such as the world context). I just feel like, as a user, there is an extremely high barrier of entry into fiddling around with the editor layout and capabilities that is preventing the less “hardcore”, yet still coding-experienced scripters / modders / programmers (who have trouble finding the several hours it would take just to get a sense of how the API would work) to begin expanding the Editor’s functionality.
If you COULD make modifying the Editor work similarly to UMG, then that would be absolutely incredible! But I’m feeling like that would be something rather difficult to do compared with just streamlining the code process. Still, a visual aid for the “Designer” side and a back-end on the coding side would be an extremely convenient way for people to mess with the editor and would like invigorate a rapid influx of new editor designs and abilities.
Whatever time/resources would go into it, the potential benefits would outweigh almost any loss since you would be paving the way for essentially massive crowdsourced development of the Unreal Engine.