Something that I thought would be cool and very useful would be a Prototyping shield that had a breadboard.
See this Adafruit quarter size board, very nice. https://www.adafruit.com/products/1608
You might not be able to have + and - rails on both sides and may have to take out a few rows of pins long ways to make it fit but you could easily put a dip chip or transistor, and anything else you could build on a board like this. I know the shield size is a bit skinny but maybe one or two holes for each stacking pin, if you needed more you could jumper it to a column on the breadboard. Would have worked great I think on the project I've been working on.
Been looking around and haven't seen anything like this that I can remember.