You could still have deeply involved missions that require lengthy planning and commitment by players. To have a join in progress style option it would require more of a random dynamic set of tasks / events that players could jump in to a server and start completing. Would take alot of work on the senario building, endless triggers and AI scripting to keep the mission going. It sounds Hoaky I know, but it could make some cool combat situations that could be fun to play out. I know I keep referencing DCS A10's MP, but that is the only thing I can think of that runs like this and works well. They set the missions up like a huge battle that runs continuously on a server. Players pilot A10s and start hitting required targets to advance the mission triggers. I've seen some of these dynamic missions go on with 3 guys or 30 guys, just a matter of who shows up at various times. Players coordinate well on TS or fly solo, it's your choice.