As you may or may not know, Survivor started in Sweden and then spred to Denmark (where I'm from) and Norway, before it made the leap across the pond and got so popular over here. The twist that I think will be next in the American version is the lone contestant that lives on his/her own until the merge. In the third Danish Survivor, that person actually ended up winning the whole thing. I know that Sweden has had put two contestants aside and thrown them in the mix later. No to be outdone, the fouth Danish survivor threw in 3 new contestants at the time of the merge. Also, both in the Danish and Swedish survivor, there has been a vote on which previously evicted contestants should join the merged tribe.
All in all, I think this type of twist will happen at some point in the US version of survivor.