Well I think this is a great step forward but it would be great if we could mix aspect ratios even better...

Consider a similar layout to OP but the landscape images will span multiple columns as well as everything it already does.

The thing about masonry is that it adapts to the size of the images. You could already do masonry using flexbox if you know the image sizes (https://github.com/hannasm/masonflexjs). Doing it as a true mosaic layout would be a step above current capabilities. At that point it's probably pretty easy to create configurations that don't fit perfectly/ require lots of empty space to layout nicely though.