I loved Total War: Shogun 2's Realm Divide Event, if it wasn't broken. I'll propose my version...
Earth Divide: Mid to late-game, once the Brain reconnects with Earth, there's a new division, no longer an all out faction war.
(Mother) Brain regains her loyalists and slowly the other factions vs. you and the alliances you've prepared.
So "Earth Divide" triggers in "OXC Factions" after you reach a certain high score or high tier level of tech. This means Mother Brain makes contact with Earth.
- 40-50% random alien factions(allies included, but lesser chance) return to Mother's side immediately and focus most attention against the human/alien retaliation, AKA you and your allies.
- As time passes, one by one, your diplomatic enemies who didn't get mind controlled at first will eventually be persuaded to seek destruction upon you and your alliance.
- Every time the Brain regains a faction, a major dice roll against you will be checked whether an ally stands by you or break away depending on*
1. faction strength(both yours and your ally)
2. diplomatic points that determine your relationship with your ally.
*After 3-5 rolls, your allies are influenced by your Braveheart(the movie) characteristics and will permanently remain with you.
Final note: Allies who leave you during Earth Divide will remain friendly towards you. They respect you, but are cowards and can be attacked later on.
Add-on Idea:
You could also make one unique faction permanently hate the Brain. They can be the Magicarp of your mod. Your starting relation with said faction is "Strong Ally". They're a defensive ally early to mid-game so you'll have to protect them from large enemy parties. They'll shine in the Earth Divide event (for balance?) and disappear late-game. They have unique technology only they can share with you, they're also focused on technology to free themselves from total mind control. They'll be immune to Earth Divide and begin focusing on warfare at that point. They won't tell you, but they're also working on Warp Gate Technology so they WILL disappear suddenly and by the end of that Month's report, you'll be notified that no trace of the Protoss is left on Earth. Hope you have some strong allies.
You'll probably be telling stories of how the Floaters stuck it out with you when all your other allies died or fled away. Floater/Human freedom!
