I just played Widelands for the very first minutes but I've got two ideas which would improve the gameplay.
The first one ist really simple: I opened a dialog window and wondered why it wouldn't close on pressing ESC. In most games this would be the common action for it. (I know, right-click does it, but it took me some minutes).
PS: I'm from Stuttgart, Germany.
The second: Why not include some description in the dialogs. For example in the building window, include some text which describes what the building does on hover. I know, there is a documentation, but why not designing a game which doesn't really need a documentation to start on?
The third has nothing to do with the gameplay itself, but could you include a dialog to change the music track? The soundtrack's not too bad, but some tracks.. I just don't like them.
I would really help in realising this ideas, but I've got not clue about mailing lists, IRC and all that, it would take me weeks to get into it. That's why I simply posted it here.
But I've got a question on this, too: Where would you place decisions of interface design? Is this programming or artwork? I think there should be some extra team for it.