Most likely it will be Sunday 6/6:30pm (in case the comp expands to 18 teams and Perth is one of the teams, hence that timeslot becomes already convenient for their matches). And also, with Ch9 Sat night game most likely having a pre-game show (hence game kicking off around 8pm), I wouldn't be surprised if the other two Saturday games on Fox will be moved to about 4pm and 6pm respectively, which gets to my next point...
Now that MNF is canned (Thank God for that), it will be interesting to see what timeslot the 9th match will fall in if MNF is moved to Sunday evening in the current 8 games/round format. The only two slots that don't clash with others would be Saturday 2pm (if the other 3 Saturday games start at 4/6/8pm respectively) or Sunday 12pm. Sunday 12pm would work if it was either a Warriors home game or in one of the suburban Sydney grounds (e.g. Brookvale, Leichhardt) in winter, but definitely not in March/April while the weather is still warm. I don't know if Saturday 2pm will get good crowds, given the situation with people working and kids playing sport on Saturday.
Given the amount Ch9 have paid, no way would Ch9 want a Sat night that goes head-to-head with Fox unless it was a 20 team comp with 10 matches (which is still at least another decade away, as we haven't even expanded to 18 teams). Until then, Ch9 and Fox will both want to host matches that don't clash with each other, even if Fox does obtain rights to simulcast all 8 matches in the round.
12pm will not happen. No way in the world.
The idea of 4 games on Saturday, (2pm, 4pm, 6pm and 8pm) does sound like it would work, but Saturday afternoon has always been hated by League fans to attend games. It was a gamble for us to back to Saturday afternoon for this deal.
8pm does sound rather late IMO but if we're going to do it at any time, Saturday night is the night to do it. (Must be a game in the either Sydney, Brisbane or Melbourne followed by a night out)
I personally hope for more Sunday afternoon football. 2 games at the same time at 4pm. One hosted by Fox, the other hosted by 9. Nine will have an exclusive game leading into their news, and Fox Sports would have three exclusive games on Sunday, culminating in the Sunday night match followed by the Matty Johns Show. (Super Sunday)
These matches would cannibalize each others ratings, yes, but that is going to happen at some point in the future as expansion comes. I'm sure the AFL have matches that are on the same time or at least overlap. They have 5 games on Saturdays.
Ultimately, it comes down to what Fox will want. I think the NRL would be ok with having another game on a Saturday. It's also possible that Nine have in the new deal that no other game can be on at the same time as one they are showing.