I chose markdown as main format for gethly.com but i needed to extend it with various features/widgets. This is where I found its limitations because i essentially ended up doing the same pattern with ::: block pattern, which you can see on the help page here https://gethly.com/page/markdown
Additionally, I have added attributes with {} for images. Overall, my impression was that BB Code might be better for extensibility of the format, despite it being essentially just a different format for raw html. MD is not perfect by any means and is essentially limited by the amount of available human-friendly characters/patterns that you can type. But it is still convenient and easy for most people. Hence, the simplicity implies there is no replacement as it is already simple enough.