No it's not what you may be thinking. Not advocating for some weird unbalanced complicated system where you combine two classes or use abilities from one class with another.
Instead, what if we had the option to pick an additional class that way may activate by speaking to class trainers.
I know many people feel with alts that a feature like this would feel redundant. I disagree.
Not everyone, such as my self, has the time to play alts or are too committed to one toon. Yet might be interested in a different class.
So why not give us the ability to pick a second class.
Would give a form of competition to ff14 since they're able to play as all classes on one toon.
Another benefit would be for players who play pure dps classes would have the opportunity to pick a hybrid class and be able to tank or heal and not be forced to play an alt.
Alts would still be useful as prof alts, classes not available to your race, extra xmog/mog runs, everything that people play alts for.
To supplement this, expand race/class combos too. But ones that actually make sense instead of combos like maghar warlock, velf/undead paladins, draenie warlocks.