I've said before that Seven is the only channel that seems to have a gap in it's schedule to accomodate Big Brother - and they desperately need a big reality franchise for the second half of the year - and next year I'd guess it would be post-Olympics, so be well promoted. Although Ten is it's natural home I do think Seven is a better fit than Nine.
Format wise although a switch to the US format would mean it's something new (though not brand new as you've had iterations of Head of House), I don't think it's so much the format that is the issue, it's the production. On paper the format on Nine actually wasn't bad with the consistency of the 5 nomination points and the addition of the Superpower, which I thought was an excellent way to introduce a weekly twist but keep it consistent under one banner. The show has to be made for Australia and I think the original format still works in that regards - it's the production and scheduling which needs to work with the show and of course I'd also argue the live feed is absolutely necessary and I'm sure 7 could monetise that with an app and through 7Plus in a similar way to the Tennis and Olympic apps of the past.