If it was one thing for unique characters... I would have to say armor variety.
Not just vanity slots, not just A or B side grade equipment but a *large* variety of similar equipment in each tier while people would strive for BiS and Vanity would help them look the way they wanted to in approaching that you would have a lot of different looks for players.

I would like to (and hope that as the game matures this becomes possible) see 4-5 styles of equipment be in the top-equipment tier. Then another 4-5 in the next tier down. This would allow for a bit more mix-and-match to reach content minimums / stat-tiers.