It seems it would make a lot more sense to just design the language from scratch, rather than try to bend Markdown to do something it was not at all meant for.
For instance, why would you WANT to have an example like this:
Welcome to London
---
Welcome to Berlin
The perfect is the enemy of the good.
You could easily borrow some common things from Markdown to make things easier, but this seems to try to force following the Markdown syntax as much as possible, even when that syntax makes no sense in context.
It is much better to invent new things for the cases that are completely new, than try to force a square peg into a round hole.
Also, if you don't like ![](), you can just use stage directions with brackets. The equivalent script will be :
(image: london.jpg)
Welcome to London
---
(image: berlin.jpg)
Welcome to Berlin
Readable but has some intelligence and decoration that is not distracting. See it enough and the syntax will become invisible over time like using punctuation.