It will be something "unexpected". The next version of AP hardware is definitely not "unexpected", it has been endlessly speculated about for months now and many predicted it was going into production weeks ago (they are wrong, obviously).
Whatever Elon meant months ago when he tweeted "...the real steering controls for the 3. It feels like a spaceship" is not "unexpected" in the sense that it has been extensively discussed for months now, but in another sense it is unexpected because no one outside of Tesla has any idea what the heck he was talking about. So it seems possible that could be what Elon is referring to.
My opinion: the "spaceship" controls that the Model 3 will offer a year from now when it goes into production (as an option of course, not as standard considering the low price point of the base version) are going to go into production in the S/X this month and will be standard. That is what will be revealed on October 17th. Elon has said several times this year that the S/X will always have the latest and greatest features before the 3 and any other lower cost models. There is no way that the 3 will launch with a cool feature that the S/X don't already have.
Of course I could be wrong...