The first two Grand Prix consisted of six tournaments, but the 2014-2015 edition had only four. Often there were problems finding sponsors and many announced host cities were changed eventually (to date, 8 of the 16 locations have been changed). The 2014-15 edition was announced late, with only 4 events instead of 6, reduced the prizes per event by about 1/3, and had no money for overall placings (as in the earlier editions). . In 2014-2015, four top 10 players (Carlsen, Anand, Topalov and Aronian) didn't participate, with the small prize funds and organizational uncertainty being the usual reasons given.
The winner of the men's Grand Prix (and sometimes lower finishers) gets entry to the Candidates Tournament. The winner of the women's cycle is directly qualified to a championship match. The women's edition has been dominated by Chinese GM Hou Yifan, who won all editions so far.
The intent for a Grand Prix in 2016-17 has been announced by FIDE/Agon, with the days already theoretically fixed to ensure scheduling consistency, but no further details have been revealed other than the existence of a bidding process being put forth to federations by September 15, 2015. This is perhaps not unexpected, as the previous Grand Prix was only announced in August 2014 (including the reduction from 6 events to 4 events), with the two initial events moved to October 2014 from the original May 2014 and July 2014 calendar dates. Agon's website lists the dates as May 11 to May 25 for the first Grand Prix leg, confirming the FIDE calendar. Most participants will be in top 25 of the world, and again the top two finishers will qualify for the next Candidates. There is no conjunction for the successful challenger who emerges from the March 2016 Candidates tournament, so top players may be unwilling to commit to the Grand Prix before this event concludes. The succesful challenger would not only be busy preparing for the World Championship, but would also have no reason to compete in the Grand Prix, as qualification to the 2018 Candidates would already be warranted (at worst) from the title match.