It would be great if the Wii U had apps like Quicken so people could do their taxes on it, or Office suite programs that would allow them to do spreadsheets and things like that. The hardware itself is certainly more than powerful enough to pull it off, and it would be a great thing for those who don't yet have a computer (they are out there).
Oh yeah, and who could forget a Mario Paint app? Make it simple enough so anyone can play with it and have fun, but allow enough complex features so it can be a competitor to Photoshop. As long as it allows file formats to be exported to SD or uploaded online then it will be awesome. The tablet controller is perfectly suited for this, so no extra peripheral would be required, but I suppose they could allow a USB mouse to be used.