Shower thought: What if, in the end of the Destiny saga.

Its revealed that Xür and the Nine are the ultimate bad guys and the total toll of all those strange coiins we toss his way goes directly in to building an army. So the last Destiny game just converts the total number of coins spent over 10 years in to a resource and says

"Guardians, you need to kill 1.2 billion Xur-like creatures who are taking over the universe. Its your own damn fault for not checking who he was before giving him this much currency!"

And then the game just shows a total kills over all players and no one gets to finish the game before we kill all those enemies!