As a new Diagram Library I would love ArchiMate. Oy shouldn't be that hard, as many of the shapes and all of the lines are already there. I personally wouldn't mind and think it might even be wiser if Miro doesn't concern itself with 'validity checking', i. e. which lines go with which shapes.
The only thing that you would have to really think about is how to find an intuitive way to handle aggregation/composition, which is what I currently mostly struggle with with my makeshift ArchiMate templates.