> Reminds me of Peter the Great's influence on Russian Cyrillic. Among other contributions, he happened to like the shape of the Latin letter R, so he just bunged a backwards one into the alphabet where it represents the sound 'ya'
Do you have a source on that? As a Bulgarian (where Cyrillic comes from) i had never heard anything of the like, and a short Google, in Bulgarian or Russian, found nothing.
IDK about Я, but Peter I definitely reshaped, along the European typesetting guidelines, some letters like lowercase a (which traditionally looked more like the Greek alpha, α), and most drastically the t (т) which for the best part of 17th century looked like Latin m. (This shape still remains in Cyrillic cursive.)
He didn't invent those shapes, though; just picked the preferred ones. In particular, the m/т distinction is still present across different Slavic scripts even in non-cursive form, because they standardized on different variants:
https://ru.wikipedia.org/wiki/%D0%A2_(%D0%BA%D0%B8%D1%80%D0%...
#/media/%D0%A4%D0%B0%D0%B9%D0%BB:Special_Cyrillics_BGDPT.svg){kind=link}
sometimes when I visit twitter, browser or whatever starts to think that Russian twits are actually Bulgarian, and this changes shape of some Cyrillic letters, making text looking somewhat funny to russian eye. T is one of them, IIRC.
Я evolved from the letter Ѧ, the pronunciation of which happened to match that of Ꙗ (that is, a digraph IA, representing the iotated A sound) in Eastern Slavic languages very early on. Its handwritten cursive version looked like this:
https://ru.wikipedia.org/wiki/%D0%AE%D1%81_%D0%BC%D0%B0%D0%B...
From there the transformation into "Я" is obvious. And it happened gradually over time long before Peter; he just happened to look at various different shapes already in use during his time, picked the one that looked the simplest and the most appealing to him, and (sometimes) did further minor tweaks to simplify.
It has absolutely nothing to do with the shape of the letter "R".
Hangul is far less capricious, though, a marvel of careful design.