Throwing this one out there, but no one thinks there'll be a father-son bonding during this "temporary" eternity?

Maybe Charles and Lelouch will recognize their differences, get together and just talk out their problems, in which the outcome ends in Lelouch being more like his father, stated by Yukimura, or Charles becoming good like Lelouch.

Maybe Marianne had tried to escape a life of Charles' concubines, and decided to seal herself away at hosts. Maybe that was why Charles got pissed and decided to get rid of her.

But since it's Code Geass, I spectate that while Lelouch and Charles are busy discussing their idealogies, Charles will reveal that he had found another way to bypass the seal exits for the Sword and destroy the world, however being stuck with Lelouch for all eternity. Then, for some plot-hole existing reason, C.C and Marianne bust into the Geass Elevator, sealing the Sword, and taking Lelouch out of there (leaving Charles and Marianne behind).

Then, as Suzaku and Schneizel start to conquer the world, now that the OBK had declared a truce, Lelouch comes back with C.C, rounding up geass'd slaves in an attempt to stop Schneizel and Suzaku. Lelouch brings Jeremiah, kills Suzaku all amidst dramatic irony that Suzaku only wanted the best for Lelouch after killing Nunally, and Lelouch breaks down Britannia.