There are those of us who like to use veroboard, at least for prototyping, want lots of IO and don't want to do custom PCB layouts for everything. What chance is there of having a Mega2560 on a pure 0.1"-pitch board? In other words, the hack-friendliness of the Arduino Nano with the power of the 2560 (or larger).
Alternatively, DIL/IDC headers for ribbon cables to break out ALL of the IOs, eg as seen on the DE0-nano FPGA board.
Either of those options would rock.