Turf Accountant to the Space-Gentry
The key plot-point of Final Crisis: Legion of 3 Worlds is going to be the fate of Superboy-Prime. What Johns (and, by extension, DC) do with the Legion is also of interest, but that's an issue that's external to the story. But Prime's story is at the core of this series. Superman's declaration that the heroes need to redeem Superboy-Prime is the main thing that separates FC:L3W from all the other cheesebag crossovers that have been inflicted on us over the past quarter-century: it's more interesting than just fighting and cosmic destruction, and it seems like a really hard thing to do, even for Superman.
There are those who believe (and not without reason!) that it's silly to even be discussing the notion of redeeming him; the idea is that his crimes are of such enormity that he can never be redeemed. This point of view is not without merit. However. First, it's entirely in character for Superman (and retroboot Saturn Girl!) to try it anyway. Second, if you don't redeem him, just what do you do with him? They're certainly not going to kill him (even if they could). They can't lock him up anywhere (even if they could), because he's better at getting out of impregnable prisons than Mister Miracle is. The only way to stop the guy is to get him to want to stop. No, he doesn't deserve it, but that's not the point.
So here are all the different possibilities, as far as I can tell, for what can happen to Prime. Place your bets!
It Is a Far, Far More Lame-Assed Thing I Do: (2:1) Of course, the most painless way for Superboy-Prime to both redeem himself and pay for his crimes would be for him to die fighting the Time Trapper, or the rest of the villains. It would be a very tidy way of ending the story. It would also be as boring as "Ninety-Nine Bottles of Beer on the Wall (Extended Dance Remix)".
Nothing: (3:1) One very real possibility is that nothing will happen to Prime. He'll get away at the end, or maybe be locked up in some way that he can obviously get out of, and continue on like he has been. This one is somewhat plausible because a lot of people think that the final blacked-out figure on the cover of Adventure #1 is Prime. Plus it fits in with DC's habit of promising big changes but not delivering. Of all the possible endings to this series, this is the one I like least: it's the one that would most drastically fail to deliver on the promise of the premise. (Similarly: it's possible that the new status quo for the three Legions will be the same as the old status quo. Bleah.)
Orange Jumpsuit Plus Flight Ring: (5:1) The most likely way this one plays out is for Prime to be overcome with remorse when he finds out that Earth-Prime still exists, and joins the threeboot Legion as their Superboy, where he will work to try to make up for all the evil he's done. It could happen. I wouldn't mind this one. A couple of things I like about it: one, it acknowledges that, as Spider Robinson points out, the word "absolution" contains the word "solution", and two, it gives the threeboot Legion a role in future stories.
Steps on a Rainbow: (10:1) He could just die in the fight. Sodam Yat or one of the Legionnaires or Kon-El or Bart (unlikely) or Superman (even less likely) could be pushed too far and, somehow, be moved to kill Prime. Might make a good story. Only problem is that the focus of the story would be shifted to that character, and that could be kind of jarring.
The Faith Option: (10:1) Prime is made to feel remorse, somehow, and says to the Legionnaires, "Lock me up. I deserve it. And I promise I won't try to escape." Nothing really wrong with this one, except that it's a bit too easily reversible for my taste.
Lost at the End of Time: (10:1) The battle against the three Legions ends with Prime being stuck at the end of time, where he will eventually turn into the Time Trapper, just as was revealed at the end of #4. As an ending, this would be tidy but unsatisfying.
The Nada Option: (15:1) In the renowned Sandman series, there's one part where Dream confronts his old lover Nada, whom, in a fit of pique, he condemned to Hell for eternity. He's sorry now, and wants to know what he can do to make up for it. There's nothing, of course; she's had centuries of torment and, at best, will always remember that. But he finds a solution: he takes Nada's soul to be reincarnated as a newborn baby, giving her a fresh start. This is also the kind of thing that can be arranged for Superboy-Prime, since this is, after all, a Time Trapper story. I'd be okay with this ending.
The Parallax Option: (25:1) It wasn't Superboy-Prime doing those terrible things all along! It was an evil entity controlling him, and all we have to do is exorcise it, and he'll be a hero again! On the one hand, it would "work". On the other, it's a stupid cop-out that Johns has famously used before.
Chronal Surgery: (25:1) Send the threeboot Legion back in time to visit young Clark-Kent-Prime and teach him about heroism, so that when he gets sealed up in the walls of reality with Earth-2 Superman, Earth-2 Lois and Alexander Luthor, he doesn't go off his chump and lose control against Kon-El and the Titans. The big problem with this one is that it would erase a bunch of continuity that DC is sort of relying on these days. I still kinda like it, though.
The Field: (15:1) This is the catchall heading for everything I haven't thought of. And it's the one I'm hoping for. I don't want to be able to predict how this damn series is going to end! Surprise me, Geoff Johns!
Did I miss any obvious ones?