Will there be a sign up sheet to play, or more of a first come, first served model?

Perhaps I may have to bring my "big box" just in case there are more people who want to play this than FFG has open copies. Though to be fair, the all expansion game isn't all that fun but playing with all the cards and an extra board is a blast.

Arkham Horror, right? I've only played the base game. Does it get more complicated as more expansions are added?

Each one adds a mechanic or two (as well as a plethora of cards, tokens, etc). I've never played with more than 2 expansions, and can't really imagine what having ALL of them at once might be like.

I've played an "Omni-game" a couple of times. It can be great, but can get a little hectic depending on what is going on.

I've only tried it once. A group of 6 of us were soundly slaughtered. It's definitely a neat idea, but it gets pretty chaotic.