A clear opt-in should do the trick. Maybe even with some percentage indicating how much you'll miss without? (e.g. not much at all if it's mostly single player, everything if it's a competitive persistent works like Eve and something in between if there are both "open" and "regular" instances)

Valve is the only organization that could pull that off, and it would further deepen the entrenchment of their position as the meta gamemaster of all PC play.