Make a Hall Of Monuments for Guild Wars 2 only. Showing off all of some one’s Mini’s and best Armor sets and Exotics and Legendary Weapons.
And depending on how many achievement points, Weapons,Skins,Armor,Crafting Materials,Mini’s etc they have they can customize their own personal HoM and can brag and show it off to friends.
Also have a reward system where for example if John the Norn makes x amount of exotics through crafting he unlocks a special legendary Sword. And these won’t be easy exotics to make.
Or you could have Sam the Sylvari who has 30 minis in her collection and all of them together earn her a special HoM Elite Mini which is a giant undead dragon that follows her around.
Just make more things to work towards. I have noticed more and more people are having legendaries. And that is fine but make a colossal personal sanctum where it would take you years to unlock everything therefore making guild wars 2 gold you unlock more unique so not everyone would have the same thing because they would have to choose what they want to focus on and branch out.
Another example would be to have a living story part of the HoM. If you did x amount of living stories fully complete you unlock a title that really stand out.
And finally back to the crafting materials. Make it so if you want to have your personal HoM look truly exotic and cool glimmering with magic and fire etc You need x amount of supplies to do so.