Get rid of any keybinding or UI convention that is there because that is the way they did it the AI Lab in 1967. Make the UI as familiar to the average computer user as possible (but keep the general design of a large rectangle of text) by using mainstream conventions (which come mainly from the Mac and Windows) for how to respond to this or that keypress or to clicking or dragging with this or that mouse button.
Inside Emacs is a cross-platform toolkit (where the platforms are MacOS, other Unix derivatives, Windows and the terminal) I would split Emacs into 2 projects: a toolkit and an app that uses the toolkit. That way, if someone wants to create an "standalone" org-mode app, Magit app or Gemini browser designed to appeal to people who do not want to spend any time learning to use Emacs the app or "Emacs the generalized interface to information", they have a straightforward way to do so. (These "standalone" apps that are as easy to learn as any other GUI app will I hope help popularize the Emacs ecosystem.)
One thing I definitely would not change is I would not make Emacs dependent on or closely integrated with a browser engine.