A critical component of AlphaGo's success is the massive training database comprising of the entire history of documented professional Go games. So while AlphaGo may play the game with an inhuman clarity of reading, it is less clear that it can strategically out-match professionals in the long term who may have an opportunity to find and exploit weaknesses in AlphaGo's process. Lee Sedol had that opportunity, of course, and he was not able to defeat AlphaGo. And how will AlphaGo improve, now that there are no stronger players from whom to train?

Will AlphaGo show us better strategies that have never been done before? In other words, can AlphaGo exhibit creative genius? It may have, but that's rather hard for us to observe.

In any case, I am looking forward to future AI vs AI games. It is still fundamentally a human endeavor.

Can't find the reference now, but in recent interviews the AlphaGo team claimed that one of their next steps would involve training a system without that training database, from scratch (simply by playing lots of games against different versions of itself), and that they estimate that it would be just a bit weaker.

Most of AlphaGo's learning came from self-play. Hence how it was able to vastly exceed the skill level of its initial training data which were amateur, not professional, games.

I don't know if it would be that bizarre. Once AlphaGo can beat the best humans on Earth, what motivation is there to keep improving it? Wasn't that the goal of the project?

Advances in deep learning in general should apply here, and there's a big motivation to keep improving that. Also, Go is popular enough that it should experience the same sort of commoditization drive that advanced Chess engines did, where Deep Blue level play went from being on a supercomputer to being on a smartphone. Then, since this approach scales up with more computing power, running a hypothetical future smartphone-Go engine on a big cluster like AlphaGo has here should put it way beyond the human level.

AlphaGo is still a monstrosity in terms of the hardware it requires. Improvements in AlphaGo will be reflected in the fact that it or something like it will soon sit on a tiny little computer near you. See also: what happened after the chess world champion lost to a computer.