Keep desktop PCs the way they are, but move all user "stuff" to a central home server type machine (or a separate drive in the desktop PC if no server is available). The tablet would sync to the PC whenever it was on the same network. So both would have
the same photos, doucments, music, movies, etc. Backups would happen automatically to a location in the cloud.
When activated, the tablet would turn into a touch surface for the desktop PC. Not sure how this would look, but basically touches on the tablet get sent to the PC for interaction. This essentially replaces the mouse.
When the tablet is away from the PC, it functions as the iPad does, with its own touch-centric apps and media capabilities, ready for the user to "consume".
Should the user choose to, the data on the PC/Server will be accessible over the Internet so the tablet would always be up-to-date. Allow remoting into the PC from the tablet in case extra work is needed on the road, but improve it so that a user could conceivably
play a 3D game on the tablet, assuming the bandwidth required is available.
The tablet itself would be about the size of a mousepad, but able to fold up to fit in a pocket.
Finally, the setup of all of these devices would be "plug it in, turn it on, answer a few questions" and nothing more. The user should be completely up and running in under an hour.