What you want IINA to do:

Under 'text subtitles > advanced > shadow', it would be nice to have the option to customize the angle of the shadow.

What IINA does currently:

Displays subtitle shadows at a fixed angle.

Why you think this should be added:

I believe default shadow angle is too close to horizontal axis and it makes subtitles a bit harder to read.

Examples of other projects that have something similar:
In VLC advanced settings;