Just like the original Anime adaptation, those who will apply as: (altering Death Note as a whole plot)
Bearers would either create a utopia of their own, controlling things out just like Light who planned out a controlled world - criminal-free that is, or a mere holder of a notebook who wants to escape death (this is an altered plot. Those who bear notebooks are given immunity over the planned out death against them).
Shinigamis would follow the order of their own bearer, following the bearer's order alone. Since there will be other perspective "Kira"s who will be also joining the rp, Shinigamis are forbidden to engage themselves with other bearers, unless, they're given so. Unlike the original story, citizens can see Shinigamis. (rest of these shall be plotted for the members' likes and preferences)
Citizens are those mere lurkers. They have their own lives. I wouldn't restrict nor intervene themselves if they choose to be a student or, otherwise, (such as detective, idol, etc.). Though, upon their deaths, they can be a shinigami (altered plot).
Things not mentioned with regards on how Bearers, Shinigamis, and Citizens be worked out as may be raised as questions.
Since this will involve killings, the rule then will allow killings given that there is a MUTUAL AGREEMENT between the bearer and the citizen. The citizen then, upon death, can transform into a shinigami, resurrect himself as another character (a bearer, a shinigami, or a citizen of his/her choice), or leave the rp for a while or for good.
Another rule made specifically is that a bearer can only kill ONE citizen after seven days of acceptance and could only kill another citizen for another seven days after such killing. I don't want a killing spree happening every now and then made by one sole person. =n=