To my understanding this is a frequent topic, so I'll be brief: because of RDM's weird hybrid structure that just can't really translate into the holy trinity system, what about if a RDM is kind of like a "role roulette" when you queue up with it?

Give RDM the ability to switch between a tank/healer/dps "stance" of some kind that makes certain abilities stronger/weaker/different altogether, and when you queue up for a group, it will randomly choose a role that's needed in that group and fill it with a RDM player, who can simply change their mode to fit that role

Just give them less abilities in each category so that they aren't super bloated. The price for playing a RDM will be that they are in "role roulette" when they queue up for groups (unless its a premade group) and they don't have as many options for each role as other classes

This would give them something unique while also fitting in with their "jack of all trades, master of none" idea, as well as give RDM a unique aspect to the game that no other class has, an aspect that fits perfectly with the holy trinity system, as well as improve queue times by adding tanks, healers, and dps when needed to any groups

Anyway, just an idea I had, ciao!