I think the GCDs are the easiest way to differentiate them, along with putting in some proper stances where appropriate.

As a short summary of thoughts, we'd have the Paladin with the Gladiator root having combos that change based on their stance, and skills that transition them from one stance to another.

The Warrior's weaponskills have a large recast difference between them (Some at 2.5, some at 5.0, etc) and as momentum/rage is built up, those recast times on the longer ones shorten while expending said momentum / rage.

The Dark Knight uses a more flexible system similar to Monk with Forward / Back transitions between them, each weaponskill similarly weighted for potency but applying different buffs / debuffs. Bloodspiller and Quietus being 'finishers' where the potency changes based on the previous 3 weaponskills. Your aim here is to get the right buffs / debuffs in place and then end it with the strongest finisher you can before the windows expire.

The Gunbreaker's rotates between 2-3 distinct combos, with Powder Charges instead being toggled on and expended to enhance them until they wear out. (The gauge would need to be expanded to hold more as well as increased generation over all).