There was some speculation it might be a 3-D printer.
Note that they're not premiering this mystery device at IBC, an international broadcast convention that's taking place right now... so that lessens the probability (in my eyes, at least) that it's going to be A/V related...
And this actually makes sense, as well. A 3-D printer that's affordable for the consumer is a 3-D printer that's going to be in lots of corporate spaces, etc.