Some observations... and the innerworkings of my brain from a man who works in TV....
The block episodes generally run for a minimum of 90 minutes (7:30-9pm) I can tell you now that the main bulk of BB will NOT be at 9pm. I see the following options.
Nine may put ACA on "Hiatus" for 3 months for BB to go to air at 7pm to compete directly against Home and Away. (30 min daily shows) and two 90 min shows a week around The Block. (Evictions and Noms)
OR
Nine News "hour" doesn't really set the ratings world on fire, there is a slim possibility that they will trim news back to 6:30 put BB in there or bring ACA back to 6:30 and BB at 7pm. (still 30 mins followed by The Block)
Other only way I can see it works aside from those two options above is if BB is split over Nine and Go! and Daily shows are on Go! and the "big" shows on Nine. (Evictions etc) with re-runs of the daily shows at 9pm on Nine.
Although....
Alex did say a while back that Nine like the idea of the "movie" length episodes for 2014 (meaning 2 hour eps). Maybe this is to make up for the show only being on a few days a week? House Rules is was initially 4 nights a week from what I remember. (Sun, Mon, Tues and Wed) and the Glasshouse season seems to be taking inspiration from HouseRules (with teams judging each other). This would then leave Thursday, Friday and Saturday for BB and then any extras on Go! I guess.
We are now at the point in Australia where people are embracing the "multi-channels". Go! can get up to 500k with certain content on. If people know what's on there they will follow, what's to say Nine aren't testing the water to see if they can draw bigger crowds to it?
Anyway that's just the thoughts and ideas of the scheduling in my brain. It has to be one of those options above for the show to fit into the ratings season with its usual 3 month length. All I can hope is that they don't make it a short season :/