GameBoy Revolution anyone? A GameBoy with technological capabilities similar to that of the GameCube, between 5 and 40GB HDD storage, and WiFi/USB/FireWire.
Large SD cardridges for GameBoy Revolution titles, along with a slot for GameBoy Advance (and perhaps GameBoy/Color as well) titles. Standard SD cards could also be supported this way, with built-in Play-Yan like capabilities (music and movie playback).
Plus, full compatibility with Revolution's virtual console, meaning you could put all your online purchased titles on your GameBoy Revolution, even GameCube titles by 'ripping' the data through Revolution onto your GameBoy Revolution HDD. Battery life would be quite short for the latter, but it would still be do-able, as there wouldn't be any disc spinning.