Looking at how the release schedule is gave me an idea. A few people have brought it up before, but Doomsday would be really rad.

So, the first film could be split up into two parts. Most of the heroes know of each other, but haven't really teamed up. Suddenly, Doomsday breaks out and begins wrecking havoc. They all team up and confront Doomsday, and try to reason with it, but they soon realize that it's just brawn, no brain really.

Doomsday wrecks through all of them, and the only one to have any amount of impact is Superman, but it takes a lot of his energy to land a single punch. Doomsday travels very fast (think how The Hulk jumped around in the desert in Ang Lee's Hulk) out of the area towards a populated area. They decide to have Superman try and hold him at bay, while the rest of the league try to figure out where it came from.

Superman has the classic fight with Doomsday while the rest of the league try to solve the mystery, and it eventually leads to Vandal Savage. He reveals that he has spent eons working on a way to make the world "a better place" and there's nothing he won't do to make the world better in his eyes. The way I think of it, like the final battle in Watchmen, with Ozymandias's explanation and the fight they have. When they are finally able to subdue Savage, they find out Doomsday has no weakness, as he is perfect. They quickly go back to Superman, who is trying his best to keep Doomsday out of a city. Doomsday gives a huge punch to Superman, which launches him miles into a city, and he's bleeding, and his suit is all torn. He realizes the one thing he's never done is completely let go. He's learned from his mistakes in MoS, and doesn't want to kill anyone.

The rest of the league tells him that there is no reasoning with it, and that it won't stop until billions are dead. He completely lets go of any reservations, and unleashes his full power into a punch to Doomsday, which launches him into orbit (or just very very far away, like the ocean). He stands for a second, and collapses. He has "died". You then have the denouement, of the league dealing with Savage, locking him away in their newly created prisons created in their Hall of Justice, made specifically for Savage. And then they have a funeral for Superman.

Then for the year between Justice League pt 1 and 2, you have each film dealing with issues now that Superman isn't there. And Shazam even is tries to take the place of Superman, but isn't quite the real deal.

In Justice League Pt 2, you have the return of Superman, and I haven't thought of it too much, but I think since it's a Pt 2, then it should still be Vandal Savage as the villain, and have some really interesting themes dealing with immortality, and what is even death.

As for casting, I think Doomsday should just be a great voice actor, as there wouldn't need much dialogue, just grunts. For Vandal Savage, I honestly think someone regal like Brad Pitt would be a really great choice.

Man, this became a lot longer than I thought, but I think that would be cool.