Yoshi mentioned something along these lines once upon a time. But this was related to the "hybrid" role/job concept that was floating around a few years ago. That idea involved equipping so many of one set of role-like abilities to determine your role (think RIFT trees).
Personally, I don't see BLU as a healer. But I definitely could see it as a job that could "shift" itself between DPS and Tank roles. I feel like it would be tied to the Role Abilities menu with just one soulstone, choosing which set of 10 role abilities you want.
It could even be the first job to not have any restrictions on gear (outside of job-specific gear), where the main stat of all equipped gear is automatically converted to that of the corresponding set of Role Abilities chosen. BLU abilities could alter as well (like WAR stances provide) so Mighty Guard could be featured as a tank stance with Tank Role Abilities equipped, and when DPS Role Abilities are equipped could be altered to be a self (or even party-wide) mitigation buff. Just have to follow the same rules that changing jobs have: not in battle and not in a duty.
Limiting to 2 roles could even lead to another job getting a dual-role treatment, allowing for two jobs to come out in the expansion with multiple role options. DNC could be a good partner candidate for this and could fill any of the 3 roles, really, but would likely go Healer/DPS to give a Tank, Healer and 2 DPS options with just 2 jobs added.
