This is how the current dropdown/nested menu looks like:

The way it works is that the navigation link block supports nested navigation link blocks. This is fine for simple dropdowns like the one above, but does not support more advanced use cases like the ones below:

A good start would be supporting an arbitrary wrapper block such as columns, or even some kind of a "box" block that would be like columns with just one column. Supporting a drawer menu at some point would be nice too.