Just found more evidence that an announcement could be imminent. This press release about Arkham Asylum - Shock Therapy's virtual reality experience addition released on the 20th of July 2016 says:
Saying "coming weeks" indicates 'less than a month'. That would indicate an announcement no later than 20th August 2016 - 3 days off the "60 days" timeline that Movie World indicated on the 24th of June 2016:
Even though this now means the announcement has been delayed by possibly 13 days, I still feel confident some sort of announcement is imminent. To publicly tease the announcement date 3 times over the past 10 months (November 2015 & June/July 2016) to allow it to be narrowed down to a mid-August announcement before nothing happening and suddenly Mack Rides crates appear 12 days after the likely announcement date seems too coincidental to me. I feel something has caused a delay in the announcement - maybe the crates themselves arrived late even though they (again, coincidentally) arrived on September 1st - an easily memorable date?
Like other forum members have said, it would now be logical to avoid the rollercoaster announcement clashing with the Doomsday Destroyer opening. There is a chance the general public could mix the two up particularly as firstly, the rollercoaster is highly likely to be DC Comic-themed too and secondly, when Movie World announced the DC Comics Super-Villains Unleashed themed area, they brought their roaming Justice League characters out for the media. They might do the same when the area opens next month. As a result of this potential mix-up, I do think, like @AlexB suggested, that a 'teaser' would instead be more appropriate now than a full announcement - as long as they clearly promote that the rollercoaster is opening next year instead of a generic 'coming soon'. If not, the general public will still mix up the two like I said earlier and hence, the announcement should then instead be pushed out to mid-November, the latest possible announcement date so far.