I looked for some eagle files for an arduino mega shield, but couldn't find any. Here's some I ripped from the Arduino reference design. It might give you a head start.
There is a (hopefully future-compatible) interface socket, which should match the new generation 4 boards. There are a couple of extra stepper sockets and a heater socket for output to a separate mosfet driver board.
PDFs attached for printing your own boards. I've attached mirrored versions depending on your method (one for laser-printer method, one for photo-resist)
My RepRap >