The Flood are a mass swarm that if you don't destroy the body can just be reanimated again and again. They are about 4,000 and growing in numbers. The reason I stuck with them being the main antagonist is that players are free to kill thousands and at a later point (spoilers) millions of them without consequence to the overall story as you are not killing the brain of the operation. 
In terms of Discord, I’m not a personal fan as a GM, as a player who is part of a team and planning moves with team members feels a bit meta. But obviously it does help.
I plan for major events to happen every weekend in which players can respond to during the weak. 

[@Rockin Strings]
That's not 100% true. OCs are allowed as long as they make sense and aren't a p*** take character. ^_^