So, I joined a group a group 5 months ago. They meet once a month, and the Dm will run a game if 3/4ths of the players can make a session. Before I joined they had a full 3 months of no playing because no one's schedules matched up. And since I've been with them, we've missed 3 months.

That is ludicrous. But apparently this is becoming more common among the gaming friends I have online. Playing continual-story games where even if you get to play, someone's character is dragged around while the player isn't there, and people have to either 1) play the missing guy's PC, or 2) find some silly reason why he wasn't adventuring on session 3 of the dungeon and yet not eaten, and then when he gets back he's missed the Story Elements.

Now, I had an idea of a way to run a game that might address this, however I feel sort of overwhelmed by the idea.

The Concept

Game Day is set in stone. It's say, every second and fourth weekend of the month, or whatever. Anyone who shows up that day gets to play.

Adventures are structured to be 1-session length, so that if the players who were present for session 1 aren't there at session 2, then there's no story confusion. For this reason, the campaign's setting allows for many smaller stories. A colony on a lost continent, a Thieves guild, the town Watch, a monster hunter guild, something that permits short missions that anyone can complete, but that can build off one another.

Everybody levels up whether they show up or not. The prize for showing up is getting to play that day, as opposed to xp.

My Problem

1) Balance. I have to be prepped so that if the Rogue and the Cleric show up that day, I have something for them to do that they can theoretically do that isn't either certain death or non-challenging it's insultingly boring.

2) Adventure design. Since any number and combination of classes could show up, how do I design an adventure to accommodate this and write them so that they're both 1-session length and interesting/rewarding?