The prospect of a Watari/Muraki pairing is only possible if the following points are regarded:
Watari is not a repressed, self-loathing, suicidal scientist
Tatsumi does not hate Watari's guts and treat him like a. shit, or b. a submissively masochistic sex slave.
Watari would not be moaning Muraki's name at the mere touch of the silver-haired serial killer. ( he's not that easy)
Watari is not capable of allowing himself to be raped
Muraki and Watari obviously have a common interest in science.
Watari is not into BDSM. ( no knife-sex, especially w/ Tatsumi)
Watari is fully considerate of Hisoka and Tsuzuki's past with Muraki
Watari's biggest concern is protecting his friends from harm
They both like tea.
Muraki would not give up Tsuzuki for Watari
Watari does not secretly lust after Muraki
Watari is not into Oriya
Tsuzuki, Hisoka, and Tatsumi cannot know about it
The Gushoshin are not flying turkeys
Kondo has no idea either
The relationship is not all about kinky serial-killer/doctor-mad-scientist-sex (that's for one-shots, people)
Watari would not give up Tatsumi for Muraki
Muraki's eye stays in it's socket
The author shall not leave vague or nonexistent reasons for a hospitalized Watari
The author remembers that Shinigami have remarkable and instantaneous healing powers
Watari does not cry like a girl
Watari and/ or Muraki do/does not do drag