Some ideas of what to do with this space:

- fully-customizable emojis (think of a RPG-like character customization screen)

- heck, why not full jpegs/gifs?

- some unicode programming script (running Doom)

- ?

That said, some very minor (HN-style) nitpick:

> Otherwise for an n byte code unit this is (5n+1) / 8n, that is 5n+1 content bits out of a total of 8n bits from n bytes. We can rewrite this as (5/8) + 1/(8n) which moderately quickly approaches 5/8 = 62.5%. It is nice that this limit is nonzero and does not depend on n.

Isn't a limit by definition no longer dependent on n?

Another one: encode instructions on how to draw the glyph into the text itself. The string becomes both the text and the font. Why not make it turing complete and as powerful/complex as TTF.

> some unicode programming script (running Doom)

I agree with you that Unicode urgently needs a scripting capability (*), but my plan was to just implement it using invisible tag characters [1] or something like that - but of course allowing a script to be written in a single codepoint is the much more elegant solution.

It also neatly solves the problem of how to write Unicode strings inside scripts inside Unicode strings and also scripts inside Unicode strings inside scripts inside Unicode strings.

(*) in RFC 04-01-2027

[1] https://en.wikipedia.org/wiki/Tags_(Unicode_block)

U+E000–U+F8FF, U+F0000–U+FFFFD, and U+100000–U+10FFFD can already provide you with your own emoji, as that range has been reserved for private use. Extending the range further might make sense if you need even more space in your program, but that's a lot of space already.

One more idea:

- Unicode-as-a-character, meaning: a whole text displayed as if it were one character, made for infinite zooming