to be fair, that is almost exactly the format i had with the Explorers of Haven RP (
link to the OP for reference), though that was in Mystery Dungeon style - players could select missions off the Bulletin Board and form a team to go at them with one or more other players. the problem is, when most people hadn't even begun their missions, i already began receiving messages from people saying one of their group members wasn't responding anymore, for instance. then others started getting bored, dropping out, etc... not to mention these were mostly beginners, so they hadn't heard of the concept of Joint Posts for example, which also complicated matters. they often didn't know how to set one up amongst themselves, which (in my experience) is difficult for a GM to assist with. so... yeah.
personally i adore the concept of Crossroads, and really want to try out a RP like that myself sometime, but i'm not sure if such a JP-heavy idea is good for a beginners RP. i do like the ease of basically being able to hop in and out, so that people wouldn't have to feel pressured to subscribe to an incredibly long, epic adventure or something.
i guess i'll give it some more thought.