I'm about to start making a handheld mouse/keyboard replacement for use with Steam's Big Picture mode because it's getting a bit annoying when a game has a setup dialog or asks you to enter some text. I'm thinking I'll use two analogue sticks and a few buttons, one stick for the mouse (obviously), the other for Big Picture-style text entry (where you tilt the stick in a direction and it gives you four letters, then you press A, B, X or Y to enter the corresponding letter), then four buttons for cursor keys (and text entry), alt, tab, space, enter and mouse 1/mouse 2. That should cover it, I think.
Alternatively I could use the keyboard from my very much stalled Apple Pi laptop and get a £60 Apple trackpad, mount both on a piece of scrap MDF and call it a day...
Edit: Well, it sort of works... I found a bluetooth keyboard so I figured I'd just focus on the mouse part, I got it to do the pointy part but the click part is a bit broken at the moment, because every click physically takes more than one cycle of the Arduino so you basically accidentally spam mouse 1 every time you click. Doing so made the Arduino IDE crash and I lost my work, so now I can't be bothered and I think I'll just play a controller-friendly game for now.