I have been using Emacs as my daily driver for about a decade. It's the closest thing we have to a "popular" Smalltalk environment. The desire to bring everything into Emacs is the same desire I felt after experiencing what computing could be in Smalltalk.
One of the really striking things that you're confronted with in a modern Smalltalk is the Morphic UI system. Imagine that, with one click, you could drop into a new window with a live inspector and a (very freeform, repl-ish that's not a repl) REPL loaded into the internal context of any button, window, pane, scrollbar, you name it that you can see right now. (Even my own vocabulary is stifled with the traditional UI/UX we have now!) Now imagine you can do that on any application on your computer. Now imagine you can drag those subcomponents around, rearrange them, drag them onto new windows, modify their code, spawn new ones, and so on all while the thing keeps running and functioning.
If you break something, it's okay. The debugger pops up and allows you to do the same thing that you did when you inspected the widget for any point in the frame in the stack trace. Yes, you can debug, live, multiple levels of your stack trace at the same time, that have live, editable versions of the code as well.
You don't have to imagine it. You can see it here [1] or just download a Smalltalk system yourself.
If the Smalltalk community focused on building some tooling to allow you do develop other languages inside a Smalltalk image based editor, I don't think even Emacs could compete.
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As far as what kinds of weird would be fun, I always find new interaction paradigms fun. That's one of the things I loved the most about Acme. It's kind of crazy how versatile and adaptable "execute the highlighted text in the shell" is as a core interaction loop. If you want to search files for something, just type `grep` in front of the word, select them all, and then you get a new buffer with the results. Which are, of course, editable and executable in the same way.
The two most common places I see experimentation in this are in Linux window managers and programming languages. I've tried StumpWM, i3, and AwesomeWM before. I wish there were more people experimenting with non-tiling things. The person behind this project is doing some very cool stuff [2].
There are tons of people making cool, small (and large) new programming languages like Idris, Janet, Lean, and Pony and lots of people still using really cool old programming languages like APL (or J or K or BQN), Forth, and Scheme (Racket is great, Gerbil built on Gambit is insanely good for a one person project). I see some insanely cool stuff about prototype languages doing staged metaprogramming (lookup Collapsing Towers of Interpreters if you want to read about something really cool, i.e. compiling out the costly abstractions in infinite towers of reflective interpreters), dependently typed experiments, and other wild shit.
Surprisingly, there seems to be a lot (sadly meaning only 2-3 actually separate movements) going on in the "OS space" too with immutable/declarative system specification. I'm referring of course to Nix and Guix and things like Fedora Silverblue and OpenSUSE MicroOS. That's gaining steam quickly. I personally find the Nix language rather fun and interesting in its own right, but I think I'm on the minority side of that.
Broadening out past that, hell I don't know. That's why I put the call out to everyone else :D
[1] https://youtu.be/QTJRwKOFddc
[2] https://codeberg.org/mmontone/mold-desktop