Once again, all that is known at this time is that Michael Myers will appear in this film and it will be the eleventh installment of the Halloween franchise, but that is about it.
Some fans have speculated that it might follow Halloween: Resurrection (2002), picking up a decade after that film, although this seems unlikely due to the poor fan reception of that film and the fact that two non-canon films - a remake and its sequel - were released since then.
Other fans have also suggested that it might continue the story of Steven Lloyd, who was the infant son of Michael Myers' niece Jamie Lloyd in Halloween: The Curse of Michael Myers (1995), but once again, this seems unlikely, as only franchise fans would likely know who Steven is and that entire storyline has been dead for nearly fifteen years - Halloween H20: Twenty Years Later (1998) and Halloween: Resurrection (2002) both ignored the events of The Curse of Michael Myers and the two Rob Zombie-directed installments were part of a rebooted series.
Another strong possibility is that Halloween III will be a direct continuation of Rob Zombie's Halloween (2007) and Halloween II (2009). While this is more likely than being a continuation of Halloween: Resurrection or Halloween: The Curse of Michael Myers, this still seems very unlikely. For starters, the end of Halloween II pretty much concluded the story of Michael Myers and, from what the audience saw, the character cannot be brought back. Secondly, many fans and critics were essentially unhappy with where Zombie's films took the series, and continuing in this universe seems highly unlikely.
Another possibility is yet another remake of the original Halloween (1978), but considering that the previous remake wasn't released that long ago, it once again probably won't happen.
The most likely possibility is that Halloween III will be a complete "reboot" of the franchise - most likely picking up after the events of John Carpenter's 1978 original Halloween and ignoring all the sequels, remakes, and spin-offs of the series. It has been stated that in Halloween III, the goal is pretty much to return to form and make a fun, scary thriller. The best way to do this would be to simply make Michael Myers what he was originally conceived to be - the Boogeyman.
Still, no one will know until a credible source confirms anything on the plot of Halloween III.