This has been done in many other MMORPGs, so it has a proven track record. My suggestion is to reward players who have accomplished certain tasks with titles, and these titles will give small stat boost.

How this can be done:
Using an example, a player will be rewarded the title "Intergalactic Karate Champion" for defeating Gurgox. The title will grant a small bonus, maybe +2 to evade. Bonus may be similar to the stat bonus that different faces give. Completing more difficult tasks grant titles with better stat bonuses.

The title will appear before the player's name.

Titles can be collected, and can be switched, perhaps with a scroll list in the avatar page.

Titles can be rewarded for: Completing all quests in a campaign, Killing a boss large number of times, PVP kills, PVP deaths, waving at devs, etc.

Titles can be hidden from screen if player wants to avoid screen clutter. (Similar to guilds being hidden.)

Some titles may be bought with plat.


-----
That is my suggestion, please feel free to share your thoughts.