For the past few years, LEGO has been hold a fan created competition called CUUSOO where submitted fan creations are voted upon and the winner's creation will actually be produced by LEGO and brought to market.
You might have seen last years winner-- currently in store-- the Back to the Future set. This year the winner is another licensed creation, Ghostbusters!!
Now, the original submission actually also included the firehouse, but since these sets are usually on the small side, we'll only be getting the four figures (and Slimer!) and the Ecto-1, which is all still fine by me!
More info as it's announced, but look for this set to be released some time towards the end of this year.