Then why not to add new skills for all classes such as:
A - "Unlock all weaponry": to unlock weapons of all classes/factions.
A.1 - 1 pt to unlock all weapons of all classes/factions, any quality.
A.2 - OR with ranks in this way: 1 - white, 2 - green, 3 - blue, 4 - DE, 5 - Legendary, 6 - Unique, 7 - Mythic
OR
B - "Unlock weaponry for a faction":
3 skills, "Templar", "Cabalist", "Hunter"
The player needs to choose which faction he wants weapons to be unlocked.
B.1 - 1 pt to unlock all weapons of the chosen faction, any quality.
B.2 - OR with ranks in this way: 1 - white, 2 - green, 3 - blue, 4 - DE, 5 - Legendary, 6 - Unique, 7 - Mythic
With a warning, the skill allows the character to use all kind of weapons but that doesn't mean those weapons are 100% compatible with the character's class.
Some issues may happen, use at your own risks.
Modifying, augmenting, dismantling unlocked weapons will have unexpected results/behaviors.
So you won't have to fix anything, the player has opened the door of chaos at his own risks.
It's player's choice to spend skill points in it.
By doing it, his character will not be optimal in his own class' skills but he will walks the path of divergency.
For instance, a hunter won't be able to use shield skills but using a shield with a gun + grenade build may be very good. Better armor and/or shield defenses.
Same with swords, that can be interesting.