The supposed "lack of predefined roles" does not come from the armoury system. It comes from the content we're now pitched against.

The armoury system has plenty systems in place to give each class a predefined role (diminishing returns on inappropriate armor, radical mana and HP differences, class affinity, class-exclusive abilities, AOE exclusive to casters).

Unfortunately when mobs are simply too weak, don't hit hard enough, don't resist enough and ultimately don't pose much of a challenge, those systems aren't tested to their limits.

The best way to solve (or at least the very first thing to do, probably followed by the implementation of more class-exclusive abilities and the nerfing of unbalanced abilities (Emulate, I'm looking at you) that allow people to fullfill other roles too easily) the problem is to increase the level of challenge posed by party-oriented mobs, and do so radically.

A raised challenge (that's needed and planned anyway) would push abilities to the limit and make the differencesbetween abilities belonging to the equipped class and those belonging to other class much more evident. And not only abilities.
Casters/DDs wouldn't be able to tank, other classes wouldn't be able to main heal and so forth.

AFTER the level of challenge is stabylized on an acceptable level we can actually start thinking on how to increase the definition of roles by adding new class-exclusive abilities and so forth.

But doing so before the mobs even pose a threat is kinda pointless.