There are some "classic" concepts that could be used as the basis for a meta-scenario, so to speak.
You have the "accidental" acquisition of information about something that's of interest to everyone which leads to several groups all taking their own paths to try to get it (and possibly running into each other along the way before the ultimate conclusion). Think <i>It's a Mad, Mad, Mad, Mad World</i> (I think I have enough Mads in there

or <i>Rat Race</i>.
You have the gathering together of people for the express purpose of finding something specific, breaking down into groups and going about things the way they think is best. Think <i>Scavenger Hunt</i> (anyone remember that movie from the early 80s?

), and things like The Amazing Race.
You have the "save the world/kingdom/town/lean-to" type of concept where there are several groups all working to the same goal but the one who accomplishes it gets the big prize.
I'm sure there are other variants on each of those, but my mind still isn't quite awake yet
Anyway, the point is that if we want this to be a large, interactive, all come together at the end sort of thing, something like what I've listed above would work very well. I remember playing in a game, years ago, where two of the guys in our group took up the mantle of GMing and we broke into two groups of 5 or 6 (yes, we had that many people in our group at the time... ahh, I miss the days of college when I would often run a game (usually super-hero, but not always) with 12 to 13 players

). The concept was that a kingdom that had been split by civil war was looking to end it by getting one of the two brothers named the true king. For that to happen the pieces of their father's sword needed to be found (it had been broken and spread across the land in the horrible fight with demons that had taken his life and sent the kingdom into civil war) and assembled so that the people of the kingdom would accept one of them over the other.
Each group had been hired by one of the brothers (or his vassals as the case may be) to get said pieces and return them to him. As thing progressed we began to find out there was another group of "adventurers" who were working for the other side and we each began to hear tales of the other group's accomplishments. Through it all we began to also hear of incursions by the demons again... which was disturbing because everyone thought they'd been banished forever.
The game was a load of fun, and it led to the climactic conclusion were we, at first, were at each other's throats (my mage with multiple personality syndrome ended up being the primary target of the other group, and when their assassination attempt failed the PC who had tried ended up losing an arm to my Fire Bolt I unleashed on him (and yes... we were using Rolemaster for the system

)) until the big evil walked through a portal and we all had to work together.
I've rambled, now (which I'm wont to do) but I think I got my point across... at least I think I had a point
