First off, I'm not saying this should be released anytime soon, but it would make soul combination more fun and unique. A great expansion idea, I must say.

Now, what am I talking about you ask? Imagine that by combining two and three souls you unlocked special abilities (whether they be passive, or actual attacks, etc) based on the them, but these abilities shifted depending on what souls you used. My ideal design would create four new abilities, one for each '2-of' combination and once for the specific three you're using. Obviously this would be overpowered to just get them all when you got the trees, so some level unlocks would be implemented at 10, 20, 30, and 50. The 50 unlock being the ability that combines aspects of all three souls and could not be attained until lvl 50. Also, you could choose which of the original three '2 pairs' you unlocked at the level, this way you aren't forced to take one you feel you don't currently need. Confused still? Let's have a little dun down.

Note: The following abilities are made up and just examples based on basic facts of the uses of certain souls. Calling them stupid or overpowered is kind of a dumb move since they don't really exist. So please, troll not ye of little imagination.

Let's say you're a warrior. You decide you're first soul is going to be a Reaver. Now you don't get anything special for this, it's your base class, you get you're own Reaver goodies. Next you go Paladin, and at lvl 10 you get an ability for having both Paladin & Reaver. For kicks we'll say something like this.

Shed the soul
30 power
1 minute cooldown
Whenever one of your Reaver's damage over time abilities expires, the target takes life damage equal to half of the total damage dealt and you are healed for that much. Lasts 15 seconds

Obviously this combines the Reaver's DoT focus with the Paladin's life damage, and of course, they both like to heal, so that gets a good spot in as well.

But let's not stop there, you still have another soul and four more abilities. So you've got Reaver/Paladin. Right now you're looking to be a neat little tank, so let's throw Warlord in as the third, now you're tank with guts to support your group. At lvl 20 you have a chance to unlock another ability. This time you decide to go Paladin/Warlord, and you get something like this.

Valiant Resolve
Whenever you block, party and raid members gain a 5% increase to their endurance that stacks up to five times.

Here we have a nice little combination of the Paladin's love of shields and blocking, to the Warlord's support aspect.

Now we have all three souls, so let's just get on with the rest. At level 30 you get your final '2-set' ability, which happens to be Reaver/Warlord.

Strength in Rot
20 power
1 minute cooldown
Whenever one of your Reaver's Damage over Time abilities expires, party and raid members get +1% to crit chance, stacks up to ten times. Lasts 30 seconds

More focus on the Reaver's Dots and Warlord's party buffs.

And finally you hit lvl 50 and get the ability that combines the three.

Vengeful Glare
30 power
30 second Cooldown
Whenever a party or raid member is dealt damage, there is a 5% chance that the target will be afflicted with Soul Sickness and the party or raid member will be healed for half the damage dealt. Lasts 1 hour.

You have a buff that affects the entire party, throws the Reaver's main DoT to trigger all DoT style abilities, and uses the Paladin to heal.

Like I said at the top, it wouldn't be something that could just be whipped up and thrown into game anytime soon, but the addition of "soul combo' abilities would make each character feel more unique.