TL;DR - I like the current UI system but I wish new players/new accounts started in a "Fixed UI Mode" that was a standard fixed ui with cycling menus similar to OSRS/Legacy mode but with a modern RS3 feel.
Longform - The current UI system is extremely customizable and I love it for that, however for new players opening and closing windows that they don't understand can be really overwhelming. Not to mention the docking system, while great when you understand it, is not intuitive.
I think if we took the Legacy UI built into RS3, and expanded it to be more useful it could serve as a good starting point. I've attached a photo as an example but this is a hack job at best.
I think having a centralized hot bar, plus the cyclable menu ribbon of the Legacy UI and the simple chatbox as a Default option for new players will really help ease OSRS players into RS3, as well as prevent brand new players from being completely overwhelmed with all of the menus.