Considering BBUSA hasn't adopted the twists which kept many traditional BB's going over the last few years I think there is still quite a lot it could do - and I haven't even touched upon implementing elements of Secret Story - every houseguest having a secret is very BBUSA!
In a way we have, BB6 was supposed to be the summer of secrets. All the Housemates had a partner but this had to remain a secret but production decided to confirm the Housemates thoughts that everyone had a secret partner.
I agree with your post, they could easily send in an alternate within the first two-three weeks by keeping one in sequester thus would not violate the rules of the game (no contact with the outside world).
The Bedsit twist is a good idea but for BBUSA the problem would be that the two in the bedsit could see the deals and strategies being made unless the producers kept changing the live feed in the House to avoid this. (But a lot of fans think that the DR provides hints to the HMs so I don't see the problem with this twist.)
My thought is that BBUSA should go evil and one of the main twists should be the fluctuating prize fund. Up the prize money to $1 million but remove $5,000 for each rule violation (like not wearing microphones, talking about production, etc.) put the Haves on a luxury shopping budget but remove some of the budget when a HM breaks rules.