(Crow walks in, looks on the floor) "Oh, there's my two cents... perhaps i'll just throw it in this here topic I stumbled upon."
Well, well, well... we have the beginning stages of a Character Tournament on our hands - always nice to see a member(s) of the MR Community chatting it up.
If I may, considering i'm on my second MR Collaborative Project, allow me to insert my insight in to this....
First and foremost - the biggest thing about setting up something like this (which will wind up being larger in scale than you currently believe) is finding a middle ground that works for all those who participate. What do I mean? In my first MR Project (Character tournament) - I vowed no character deaths, because there were some who weren't comfortable with that - although they wanted to participate. Obviously this is an easy thing to accommodate, but it's just an example - if you garner interest, you will find that your original concept may change drastically.
Next thing: Artists... Mmmm... This is a general lesson learned - Artists won't really have an interest in drawing anything they aren't (themselves) interested in drawing, especially if the words "Exposure" "Cool" and "Unpaid" are thrown together. Now, you may be able to get artists without much experience, but to think that someone who can make money for their art, is going to willingly draw other characters (especially when it's somewhat noticeable that one of the main purposes of this is for free work) is a long shot.
Neeeext: A Leader... multi-writers... if that route is taken, your project will most likely never reach its conclusion. Personal experience, I've written the battles, come up with the stories, invested in the character interactions and relationships... all of the decisions involved; as it pertains to advancement of the project, has been mostly me. And in truth, in order for your project to work - you're going to need 1 Writer, a max of two, if they're on the same page. Now, of course, you can implement things such as Character Moments and offer opportunities for individual collaborations and whatnot, things to get some invested participants who would like to write, involved. But other than that, to think that all the participants are going to find the time to write and write some more... bluntly - no. My first project (as well as my second) in the beginning, has much participation, but as the time goes on - and your story forms, people write less and less - to the point now, where I post and it's almost exclusively me.
Let's see... what else... oh - a story, it's importance: Large. Why? Because it was suggested that this would essentially introduce the concept of a connected universe - well, if that's the case, then a story is essential.
Lastly, I saw mention of trying to make this different, otherwise it'd just be another project like Crow's (That's me). Truth is... no matter what approach you take, if you're going to do something like this - it's going to be similar to what I've done. For me, I based my concept off of Galactic Raiders, which was a project that was undertaken by Roshiro Byakko, with aid from me. When Roshiro left the site, I took the initiative and went ahead and crafted a world that could house all the characters of MR, and then I went about crafting a Trilogy that would allow for participation from everyone. It took me 4 months to plan out Project 1... FOUR months, and it was a simple character tournament, Project 2 took me another 4, and Project 3 may be another 4... fact is, there isn't much diversity to be had when it comes to an MR collaboration - it is going to be what it is going to be: A Collaboration of characters - the way you make it unique isn't by having artists draw characters, or by teasing death... you make it stand out by putting as much effort and care in to it as you would your own separate story. I put as much attention in to my MR Projects, as I do Society X and Amnesia... it's a must if you want this to succeed.
Best of luck to you all on this. If it works out, and I hope it does, then that's awesome - i'll look forward to reading it. If it doesn't, and you all have an interest in being a part of something like this - i'll have applications open for Project 3 within a month or so.