Recently while implementing the Mongolian script, I thought, maybe vertical
text support should be added to Emacs too.

Vertical text is used in scripts like the CJK characters, Mongolian, Phags
Pa, Ogham etc.
Scripts like the CJK characters do not need to be rotated when written
vertically and can be written either from ltr or rtl,
while scripts like Mongolian are rotated and written from left to right.

So my questions are: Should this be done? If so, where should I start.
How to correctly mix non vertical and vertical scripts in the same buffer,
if it is even possible.
Should this be a minor mode only?

Here are some reference links:
This UAX#50 defines default orientation for various unicode characters:
https://www.unicode.org/reports/tr50/tr50-26.html
This website defines orientation of all unicode characters:
https://www.unicode.org/Public/vertical/revision-17/VerticalOrientation-17.html

Robust Vertical Text Layout: Using the Unicode BIDI Algorithm to Handle
Complexities in Typesetting Multi-Script Vertical Text:
https://www.unicode.org/notes/tn22/RobustVerticalLayout.pdf

css-writing-modes-4: https://www.w3.org/TR/css-writing-modes-4/

OTF Tags for vertical text:
https://docs.microsoft.com/en-us/typography/opentype/spec/features_uz#tag-vert
https://docs.microsoft.com/en-us/typography/opentype/spec/features_uz#tag-vrtr

This is what the UAX#9 says about bidi text in vertical orientation:
https://unicode.org/reports/tr9/#Vertical_Text

A website that has vertical text orientation: https://wikibilig.mn/

I do not promise that this would be completed, but atleast I can try.
Thanks.