I think I'd be siding with Light, actually. Mirai Nikki's about trying to survive while offing others. If all the players are working as a team to defeat NPC/PC villains, how exactly does the rp differ from any general "let's form a group and go after the baddies on an epic quest rp!"? Not to mention that it would make no sense, plot-wise. The diary holders are supposed to kill each other, and only one can win. Why would a group of them team up to kill off the other half? Why would this other half be "bad guys" any more than they are? We would be the ones doing the killing, after all. And if out characters have no problem offing those other people, why wouldn't they backstab each other?

Not to mention, it would take away the constantly lingering threat of your character dying if you/they make a mistake. Mirai Nikki without the fear of death is... kinda bland?