As for new corners, I feel the four main lines are a "must have". The mountain division would be a nice addition, as there are a couple of members who would want it and it would give us the opportunity to make the scenery a little more "3-D".
As far as structures go, I'd like to recycle the farm scene on the 4' corner if at all possible as it's a nice scene. I also have an Atlas station kit with platforms. If I recall correctly, John. V. gave these to me a few years back for use on a module. That would take care of scenes for two corners. I'm sure I also have some other structures I can spare.
I have enough scrap syrofoam to probably cover the scenery for a corner or two. In addition, I've got a couple of boxes of old trees I got at the GTE years ago. A little work with glue and ground foam..