I really like plantuml and so I want to give you some ideas for improvements.
I would like to be able to "delay" a change in a robust (see 'dval' in the picture below)
Other nice features I would like:
-Coloring parts of parts in a concise participant (maybe using # like in other diagram types)
-make the space under the line of a robust participant not to be filled (maybe specified by skinparam)