You should have a Plot Summary that explains how the game is going to roll, so the players who plan to join know the basic idea of what they're getting into. Then, you should have some rules for what you want and don't want in the RP. Finally, there has to be an understanding that no role play is perfect and that people will quit for numerous reasons and some of those reasons don't ever involve the RP at all. This allows you to weed out the dedicated players - those who decide to remain - and you can keep recycling players in that way. You know your dedicated players will keep on rolling and as you continue to cycle through players, eventually, you'll have a full group of dedicated players.
My opinion only, I don't think your RP will require too much GMing. All RPs, always need the GM or someone who knows the story to assist in moving it along and even placing before players different situations. I might suggest that if you control when the Negaverse sends a monster, then it will keep players from creating their own points of action spontaneously. It will be a slightly controlled game that those who are in the area where the Negaverse spawn is, can choose to get involved and those who aren't, stay out of it. I'm not sure if everyone agrees with me, but a gang bang of heroes on one enemy always was a huge turn off to me lol.
If we had an OOC discussion before the action proceeded, then we can discuss who is going to get involved and who isn't. Oh yes, another thing because not everyone posts as much as everyone else. A rule for posting amount might be good. There were numerous situations where players who write smaller than others will post a ton of posts in one full day and so the players who write lengthy or who aren't as active, when they return, there's about 1-3 pages of IC that went by. I hope there is a post limit per day because seeing an RP go ahead of me makes me quit all the time. It also makes others quit at times. A one to two posts per day would seem like a controlled one, but whatever you decide. I'm only making suggestions.