Hm, after listening to a few more videos citing more accurate translations at work, our understanding of the Outer Gods seems to be a bit off
first 6 minutes of this video roughly summarize it, but he has a whole hour long one where he spoke to translators to get a more insightful look at the wording and how the Japanese community understands them
tl;dr
The Outer Gods are not "Outer" in the Lovecraftian sense strictly, they're called that because they're outside the current system of the Erdtree, but they do seem to actually be creations of the Greater Will and more importantly, they are a part of the world of Elden Ring. This is why for example, the Rot Outer God actually is sealed below the Lake of Rot, and the Formless Mother is deep beneath the Earth. This also explains why the Greater Will isn't ever referred to as an Outer God, and it makes Hyetta's dialogue via the Frenzied Flame, where the Three Fingers assert that all things, seemingly including the Frenzied Flame, were made by the Greater Will, and the Frenzied Flame wants to return everything to the oneness that existed before reality was made.
There's a lot more supporting this interpretation cited but those are the broad strokes. This does not mean they aren't still abstract or cosmic in some form. The Rot God still obviously affects the world despite being sealed away, and is also still more powerful than the Demigods are, which would include the likes of Malenia, Radahn, etc. They just aren't of the cosmos or another plane specifically like we thought they were, they are fundamental forces of the world, closer to abstract or conceptual beings than anything else. He does argue the Fairy that aided the blind swordsman in sealing the Rot God was another Outer God but I'm not entirely sure why that is, although the Fairy and it/the swordsman's power is certainly never really explained.
Interestingly, considering the Greater Will is likely not an Outer God, you have four named Outer Gods
God of Rot
Formless Mother
Frenzied Flame
Dark Moon (not specified but almost certainly an Outer God and not a "God" like Marika, the Fell God, or Placidusax' consort)
and one unnamed fifth (God the Deathbirds worship)
Miyazaki has been using this element since Dark Souls 1, 4 Lords and 1 hidden 5th (Furtive Pygmy) and again in Dark Souls 3 (4 primary Lords of Cinder and Ludleth, another Pygmy, as an obscure fifth). The DLC seems like it is going to deal with afterlife or spirit realms (makes sense as Torrent, Ranni, and Melina are spirits), possibly the Helphen, so the fifth unnamed Outer God could play a role?
