Take a look at the latest Notcurses demo video to see what that library makes possible:

https://www.youtube.com/watch?v=dcjkezf1ARY

https://notcurses.com/

not sure how you define modern but things like neovim, neomutt and ranger come to mind.

I enjoy r/unixporn (it's actually sfw), they LOVE console apps there, everything from music to email to file manager to everything console!

There are also a few modern terminal emulators that are doing some pretty fun stuff with the terminal, some examples are alacritty, kitty and hyper

My personal favorite is musikcube: https://musikcube.com/

The mouse support is excellent, assuming your terminal emulator supports the features it needs. (Alacritty does, but iTerm2 doesn't seem to send right clicks to the program.) Pretty much everything can be clicked and right-clicked, you can even scroll (although I find it scrolls too fast to be useful).

I listen to most of my music through musikcube and ncspot https://github.com/hrkfdn/ncspot these days.

I don't know if it counts as rich, but I rolled my own terminal UI in my email client meli [0]. There's a web demo here: https://meli.delivery/wasm2.html that is the client compiled to webassembly and the ANSI escape codes translated to equivalent SVG drawing so that you can try it on your browser. [0] https://github.com/meli/meli/

take a look at growlight: https://github.com/dankamongmen/growlight

VisiData is one of my favorite TUI apps. It's a handy tool for working with tabular data, like CSVs:

https://www.visidata.org

tig: https://jonas.github.io/tig/

I love mu4e. I don't know if running in Emacs qualifies as "terminal GUI" though.

https://www.djcbsoftware.nl/code/mu/mu4e.html

It certainly qualifies. TUI stands for text UI not terminal UI

The most modern terminal GUI (as for 2021) would have to use React: https://github.com/vadimdemedes/ink