There’s no need to explicitly add the height and width of an image in Markdown. Use a server side script to get the dimensions from the image file and add these to the HTML after it has been converted from Markdown. This will make the Markdown much simpler to read and easier to write.
As for classes and IDs, how about some specific real world use case examples? I provided a solution for image alignments in 266 that does not extend the Markdown syntax. I would be very hesitant to add IDs to Markdown as you’re moving into presentation territory, the realm of CSS. There’s often a better way implementing advanced features that does not clutter up the content with metadeta.