There are portable SNES and NES systems that are very large in size, due to the big cartridges that they cater to. One cool thing about these systems is that they can be turned into a normal console, just add some controllers. You even have the option of connecting them to a TV.
I would like to see a Game Boy that is similar in size to those aforementioned systems, with the same controller and TV functionality. The 5-inch screen alone would be a worthwhile selling point for many gamers.