Rocks fell. Everyone died. It was a happy ending. As for what happened after Kallen ran to school with toast in her mouth, let's leave it here. Realistically, there would be a horrible bloody battle for world supremacy, and all Lelouch's machniations would be for naught. But realistically, giant robots can't exist
, and neither do immortality, magic powers, or girls with boobs as large as their heads. This is my theory about Lelouch and all that: Insanity runs in the family. Notice that his father and mother apparently have had a years long plan to do some mystical gooblygook about Raganorok and connections or whatever. Note that Lelouch's brother who is named after some sort of German food thinks he can create world peace by threatening everyone with magic bombs. Heck, even Nunally gets into the action with the exact same idea as Lelouch.
Also, I think that Lelouch was always emotionally immature. He had some talents, but he can't really connect with people. It's not just the power of the king that isolates Lelouch. So he probably wouldn't be able to think realistically about the geopolitical circumstances. He's a child inside, and his plan is what a child has come up with. He didn't think beyond his death since he still have the egocentricity of a teenager.
Of course I'd like to note that a story arc in which due to hubris, a single man gains the world and loses everything that matters, and then sacrifices himself to save the world, passing his burden unto his best friend is a tried and true one for a reason. Code Geass just hams it up.