I don't think there is too much confusion here. There already exist attack powers that don't require an attack roll.
Look, the PHB says that "Most attack powers that deal damage require you to make an attack roll." That indicates that not all of them do. Magic Missile is an attack power - it triggers anything that triggers off of an 'attack'. It does not require an attack roll - it does not trigger anything involving an 'attack roll'.
Rain of Steel doesn't trigger a fighter's mark because it doesn't target an enemy. It targets the character, giving them what is essentially an aura of damage. One could probably argue that activating the stance would break Greater Invisibility, but if you activate the stance first, dealing the damage wouldn't.
In this case, though, you have an attack power that targets an enemy. That's an attack.