There are many possibilities for future Elden Ring DLC, including the Greater Will coming to the Lands Between. Although it's not a certainty, the entity's importance to the plot makes it strange that it's never shown, so it could be included in an upcoming update. Even if Elden Ring DLC without the Greater Will is announced, players shouldn't get discouraged.
In Elden Ring, the Greater Will chose Queen Marika the Eternal to be the bearer of the titular Elden Ring. After the murder of her firstborn son, she defies the Greater Will and shatters it in an event appropriately called "The Shattering." According to Elden Ring's lore, Marika is the ruler of the Lands Between, but she falls from favor after destroying the ring and is imprisoned in the Erdtree. She presumably dies when it is burned during the events of the game, but the Greater Will remains.
The Greater Will is extremely powerful and kicks off the events of the entire game - but in a mostly indirect way. If it is in future Elden Ring content, then it would likely have a much more important part to play. If it's directly involved in Elden Ring DLC and not just indirectly, as it is in the base game, then odds are, players will be able to fight it. And even if they're not supposed to, they'll most likely try anyway.
The Greater Will In Elden Ring, Explained
The Greater Will is one of Elden Ring's "Outer Gods," and the creator of the Elden Ring and the Erdtree, but it's never fully explained in-game; even Elden Ring's canon ending doesn't reveal exactly what it is. Its motivations, as well as its interest in the Lands Between, are also unknown. It's presumably as old as the world itself, possibly even older, and wields more power than players could ever dream of. What is known about it is that it brought the Tarnished back from exile in hopes of one of them restoring the Elden Ring and becoming the new Elden Lord.
If the Greater Will did come to Elden Ring as DLC, then it probably wouldn't come alone. As such a powerful and important being, it would likely have its own questline, perhaps even an entire sub-plot, dedicated to it and its machinations. A great way to introduce it could be the Perfect Order ending. Every Elden Ring ending has its advantages and disadvantages, and Perfect Order's main draw is that it locks out the other Outer Gods from the Lands Between, preventing them from interfering in the lives of its citizens. It could be a great segue into the Greater Will's arrival by having it break through whatever is preventing it from operating in the in-game world.
And if the Greater Will does come as a part of Elden Ring DLC, then it will probably be a boss battle. FromSoftware games are famous for their tough boss fights, and this could be the hardest in the studio's history. As a literal god and the creator of the Elden Ring itself, players would probably not find it an easy win. Even beating Malenia, Blade of Miquella, might be easier. The Greater Will may even be a joke boss, meant to one-shot any Tarnished foolish enough to challenge it no matter how strong they are. Elden Ring has a lot of potential with the Greater Will, so hopefully, it lives up to it.