I re-iterate that the current aspect ratio is wrong. Atari pixels are 0.9/1 and anyone who cares about round objects looking round either when developing in Hatari for use on target, or for watching stuff made on target long ago on a modern monitor, you _need_ to change the aspect ratio.
"and later on the 16-bit Atari ST (low-resolution 320×200 pixels) each pixel was roughly .9×1 As such, any graphic drawn to look correct on such a display ends up looking slightly distorted on modern system with truly square pixels. "
Hatari is meant to emulate so, IMHO, if there should be any settings dealing with aspect ration it should be to scale to 0.9/1 ... with SDL2 and most people running a multiple of the native number of pixels this will look quite good, mostly. If someone needs pixel-perfect, but with the wrong aspect ratio, that's another setting.
With that said, I agree that the fixed options of 0.9/1 and 1/1 are better than completely dynamic.
/Troed