It'd be nice to have a way to, inside the transformation settings for a source, be able to tell OBS "Hey, I know you're capturing a 16:9 display, but the content that's being captured is actually 3:4 aspect ratio. Grab that from the dead center" and just get quick results rather than have to hand-tweak things. You could have presets for 4:3, 3:4. and 16:9 (in the case of where you have a 16:10 monitor but are capturing 16:9 content)
It's probably a bit more complicated than that since you'd need to specify where the starting point is on 16:9 (is it capturing from the center, the top of the screen, or is it starting from the bottom and there's a Steam overlay across the top of that 16:10 space?) but I'm pretty sure there's got to be some ways to calculate this stuff and automate most of it-- or at least offer a UI to simplify things as much as possible and then let you hand-tweak it to perfection after that.
It's probably a bit more complicated than that since you'd need to specify where the starting point is on 16:9 (is it capturing from the center, the top of the screen, or is it starting from the bottom and there's a Steam overlay across the top of that 16:10 space?) but I'm pretty sure there's got to be some ways to calculate this stuff and automate most of it-- or at least offer a UI to simplify things as much as possible and then let you hand-tweak it to perfection after that.