So besides the First Days I've created basic outlines for the three follow-up stories.

Canterlot: Exodus: The population of Canterlot along with survivors elsewhere leave Equestria on ships, and roam endlessly, eating mostly Chlorella algae. This story will have quite a bit of sex in it, most of it will be consensual, and a lot of it will be same-sex.

Canterlot: Redemption: The Army returns full-force to take back Equestria.

Canterlot: Looking Back on the Cost of Victory: A recap of the whole invasion chronicle, and the after-effects.