If I can get all the people, including multiple GMs, I'm planning a massive one shot crossover. It centers in the Alien universe. Skynet has built a new time as well as dimensional projection device. Using technobabble sensors it finds evidence of a "perfect biological weapon". Of course it sends some Terninators. To Garrison System Upsilon. Predators are drawn to the coming conflict.
Now my idea is using a play by post format for the overall game. I know a site where I can strictly control who sees what thread. With an agreed upon post rate, you could keep interaction between groups at a steady rate.
I've mentioned different groups, my idea is to have this be a massive pvp insanity. Colonial Marines, Scientists, Corporate Merc, Resistance Fighters from Terminator universe. Not to mention players for the Terminators, Predators, and Queen Mothers.
Frankly it's probably a pipe dream. Just getting that many players together would be hard. What would make it massively more fun would be for players NOT to know what other factions and enemies there were out there.