Even if there are several portable Sega Genesis on the market, they are all emulators. It would be great to build a portable version using an original one instead 
and make the first Genesis Nomad DIY clone 
The tricky part is most likely to convert video signal to an LCD, but the FPGA work with the giant GameBoy could be reused.
Plus, Genesis cartridges are quite big but if there is a way to optimize the board it might be relatively easy to make a not-so-bulky case