In my previous theory (http://www.whoisredjohn.com/See-a-theory-Red-John/3591) I analyzed some facts we were shown during all 5 seasons. Now I'd like to think on the possible trick which is waiting for us in season 6 made by Heller and co..
By saying trick I mean that probably we are not going to find out that RJ is just one of seven. I think the solution will have some complicated structure.
Here are some possible tricks we may be suggested.
1) RJ is just 1 of 7 (a very simple structure), but the trick is some rotation of roles. I mean we could find out that actually RJ is not as much evil as we used to think and PJ is not as much good. If this is true, we have two scenarios. The first is RJ seeking some revenge (for example Harker could be blinded by Janes' manipulations). The second line is RJ killing bad people only. Actually the majority of victims are not saints, some women could be revealed being bad, and the only one is PJ's daughter who was absolutely innocent (maybe we could think a chance of her being alive?). Another argument is RJ trying to stop his activity (by making PJ kill Carter). From this point everything was ok, as RJ stopped murdering until PJ manipulated RJ to kill Panzer and so reviving RJ.
2) RJ is just 1 of 7, but a pair of suspects might be excluded from the list in the beginning episodes of the coming season (some could die or something else). So we'll stop suspecting somebody, but later it could be revealed that the one is actually RJ. So we must wait for the first suspect to "die".
3) Still I can't fully understand the profile of RJ. He is said to be Moriarty, but some steps like direct murders, actions with blood and so on don't fit in Moriarty profile. So the trick may be RJ not being the highest killer at all. I mean there could be a person who is directing a lot of people and RJ is just one of them. So RJ is a very bad guy, but he is not "the final boss". By the way, this could be a possible setting for season 7 (after catching RJ in season 6).
4) I know what Heller said, but let's agree that the way he said the list is real is not the same to the way he said PJ isn't RJ. I mean we could be 100% sure that PJ is not RJ and we could be 95% sure RJ is on the list. So this may be the trick. The list could be real but not full, in which case Mashburn would be a very strong candidate (we shouldn't forget his close relationship with Lisbon). I'm still thinking about Roy Carmen being somehow involved (probably the only character who could know about PJ's happy memory). May be Carmen told Lisbon about the memory, and Lisbon told Mashburn.

There could be some other tricks beside the listed above, but I'd bet on the 3rd line (probably with McAllister).
Some words about the list. Let's imagine you were offered to guess the list of 7 suspects before it was revealed in the last episode. A lot of people would list Kirkland, Bertram, fans' favorite Partridge, certainly Stiles, Haffner was also very suspicious. Less people would list Smith (but some for sure would, I believe). Would anybody list McAllister? I think no, because the rest six were somehow suspected, and when creating a list the scenarists could point as 7th somebody more "RJable" (almost anybody from characters listed on this website in Suspects section). So what are the origins of the idea listing McAllister? I don't think they couldn't find anybody and just started to watch episodes from the beginning until finding McAllister shaking hands whit PJ.