That could be an option, but creates a whole lot of other issues - especially for smaller images!
You could create the image container <img src="..." /> for example with the following CSS settings applied to it:
aspect-ratio: 16 / 9;
width: 100%;
max-width: 100%;
object-fit: cover;
object-position: 50% 50%;
That would render every image in the aspect ratio of 16 by 9, and would stretch over the complete post width, while centering the image in this container. To see the full image - when the original is not of a 16 by 9 aspect ratio - you had to implement something like a lightbox or overlay! This also could result in "cutting of heads" from images.
But do you really want that for every image posted? Think of small/little GIFs or other small images which would be stretched out over the complete post width. I don't see that as a viable option.
Another possibilty could be, while saving a post or adding a image via editor to determine it's aspect ratio and saving it as attribute to the added image. I don't know this is possible via Markdown - BBCode might do it. Determining the aspect-ratio might be possible via JavaScript in the Frontend or on the server-side while saving a post (server downloads (image)-resource, determines aspect-ratio, saves it as attribute). Saving this meta-information is probably an issue that had to be implemented to TextFormatter first.
Also the width of the embeded image might be useful, to render it out with max-width to avoid stretching out smaller images over the whole post-width.
For responsive websites you always want to work with aspect ratios and not fixed widths or heights.
