Keep in mind how we only got the first trailer to go by, so nothing is final around here, just speculation
It should be fairly obvious how PoE always had it's fair share of Lovecraft, like the unknown, incomprehensible nature of nighmare/the Beast, which kind of ended with Sin revealing to be it's creator.
But the recently announced Elder brings those themes back stronger than ever, and is even very specific in it's similaritys to a specific outer god.
First off, the general description of "an elder being" which is threatening "our sanity" should ring a bell in anyone familiar with the Cthulhu mythos, as well as the tentacle themed beings it spreads around the Atlas.
But it gets even better: It has a lot of references to Yog-Sothoth, an outer god which is direct offspring of Azathoth and the most knowledgable being in the Mythos.
The Shaper calls it "knocking at the treshhold", which is very similar wording to "The lurker at the threshhold, a name associated with Yog-Sothoth. Since Yog-Sothoth is a being not able to manifest in the physical world (as of yet), the elder could be a form of it that has regained that ability, or is starting to do so
Notice how the elder is a strange, deformed, but somewhat humanoid being behind a cloak and mist. In "Through the Gates of the Silver Key" (Lovecraft & Hoffman Price) the protagonist encounters the outer god in the "timeless halls beyond the Gate of the Silver Key", opened by Yog-Sothtoth himself, which has taken the form of a "silhouette of a man behind a strange, shimmering veil"
Yog-Sothoth is said to be "coterminous with all time and space", so it certainly has a great influence, maybe even omnipotence, over it. Furthermore, "those who seek to interfere with space and time end up with Yog-Sothoth", so it is probably Shaper's own fault that the Elder has awoken.
As a weaker point of evidence, it is known as "the Gate, the Key and the Guardian", "Guardian" of course rings a bell as the beings who protect the Shaper and those who the Elder will summon, with "Key" being mentioned in the Shaper's line after defeating one of the Guardians: "This is the key to a crucible, that stretches the sanity of the mind"
As a last question, was this all intentional, GGG, or is this just me reading too much into coincidences? If it was intentional, great work in representing one of the lesser known creatures of Lovecraft's writings, you nailed most of it.