[@Holy Soldier] really insightful post on crossover roleplays, and even roleplays in general. I find this:

[quote=Holy Soldier]In my crossover game, we’re currently finishing up a mission in another universe, but when we return to the main universe, there will be a period of sandbox. This period of sandbox allows those writers who can interact with others and carryon their stories do so. For those role players who cannot do this, I will be making GM events that they can participate in for XP, prize items, etc. It keeps the game active and moving and not stalled. Once those events are done, we’re welcoming in the new recruits/players and going on another mission.[/quote]

exceptionally helpful.