GnomeBob
I actually think, that Ray has the biggest chance of awaking his aura naturally . He is already very familiar with meditation , and the "Nen of Flame " is a precursor to learning Aura (it also strengthens will , which will be useful in one piece Haki training ) . Ray can already sense the aura through the Force , so awakening on his own would be much easier if he knows how . That's why the body type Mc chose is hard to train with but has cumulative returns , especially with the order Mc trains them . As a side note , does anyone else think that Killua was supposed to be "initiated " when he was 8 years old , but he never fought in 200th floor , just reached it . That would explain why all his other know about nen but he doesn't .
I wonder what you have planned for ray's hatsu. an idea I just had is his hatsu os something conceptual but simple sounding, like it lets him feel if things are balanced, or how to balance something. it could be anything from a shelf to a situation to how to balance the odds in a fight or how to balance his skills. I'm sure you have something planned but theres an idea you can work with I guess