But is this a fundamentally bad idea, or is it, like kerning in general (which can require thousands of pairs to be defined), something that just needs more work?
It's not like this is limited to just a few pairs of letters like 'mi', or just pairs - if you've seen Fontemon https://www.coderelay.io/fontemon.html you know the sky is the limit for what the rewrites can do!
It's a fundamentally bad idea. I think for anything to look monospace for me the following two properties are non-negotiable.
1) Horizontal spacing needs to align vertically between glyphs of the same letters on separate lines. This is the example I give elsewhere of a letter "i" being in a very slightly different position than the same letter just two lines above it. I personally find this extremely disturbing and unpleasant to look at in monospace.
2) The glyph of a letter needs to be the exact same between instances of the same letter that are in any kind of visual proximity. This is the "gimme" example the parent of your comment gives. AnyThinG ElSe LoOkS lIkE jumblycase. Which I think we can all agree is just plain horrible.
> It's not obvious to me how any further work could fix it.
In "gimme" you could have a rule that when any character is next to itself both occurrences must have the same width. Wouldn't that fix the "gimme" example?
What about distributing the space amount more that one letter, and ensuring no two of the same letter are ever different. You'd allow things to get out of sync and not be on a perfect grid, so long as you could align everything at key points where a line has the same character on two successive lines, or some other case where you know you need alignment.
Everything else would just look like a proportional font.
Maybe it would influence coders too much and subconsciously make them choose names that line up well though...
It's not like this is limited to just a few pairs of letters like 'mi', or just pairs - if you've seen Fontemon https://www.coderelay.io/fontemon.html you know the sky is the limit for what the rewrites can do!