I would like to propose a non equipable Legendary series of Artifact.
Whenever an artifact is obtained in any of our games, the loot table would have a very very small chance for the reward to be a Legendary card of sorts instead of an equipable item.
These collectibles would then be able to be used in a mini game that would have rare and exclusive prizes.
In one of the social hubs, which I assume we will have, have a Tavern/Stadium/Area for a mini game, in full view of all, with leader boards and prizes that can only be played with these exclusive cards.
In many online worlds collectible mini games, within the game, is hugely popular and gives your Avatar a goal, a sense of progression and exclusivity.
Various past video-games have have added similar mechanics over the years to great success. Perhaps one of my favorites was the Mini card game in final fantasy 8. Or perhaps more recently the Nordic rune game from assassin's creed Valhalla. Or for the the WoW players out there think about the joy of hunting down that last Darkmoon Fair cards.
Often these mini games end up with a cult following of their own. We could also use it as a control to incentive where users visit