1 prequel called mirai nikki mosaic
Mirai nikki nikki mozek is a Manga series based around minene sama. It is pretty interesting. It shows how minene got trapped and also why she wants to become god. Not to mention it explains that the 12th actually captured her in order to get in the religion group led sejak the 6th.
2 mirai nikki As anda all know mirai nikki is focused around yuno and yuki. It is about the 12 diary holders trying to become god. A sad Cinta tale it is at the same time horror and confusing.
3 mirai nikki paradox This one is based around murumuru and akise. The plot is yuno doesn't exist and yuki got hurt therefore murumuru and akise take their places. Murumuru tries really hard to persuade akise to follow her lead so the future won't changa. However once they reach the 6th he rescues her and the future changes big time.