The DLC possibilities are literally endless. There is so much unexplored and untapped potential, lorewise. Fromsoft can expand on any of the Outer Gods and make it a worthwhile story to delve into. They can even go to the pre-Greater Will/Eternal Cities/Numen pre-calamity era before aliens went Armageddon on them. Or they can do something with the present tense, like how did Farum Azula come to be? It's supposed to work as a safe haven for Marika's shadow to hide Destined Death, but still it's pretty unexplained what all these dragons have got to do with it.
If they do something, I have 20 euros on the middle of the ocean where all the Divine Towers meet.
But because they can, it doesn't mean that they will. I'm still mega salty that they didn't do anything with Sekiro when they could have deep-dived into Genichiro and Tomoe. They were probably too busy with Elden Ring.
I wanna see how they develop Astel's gang of aliens personally. While I was reading the fextralife wiki, one player comment was pretty interesting. He mentions that the rememberances are tied to colors resembling Outer Gods. For example, anything resembling the Greater Will is golden, while the outer space/moon aliens have the same color as Rennala. There are even some like the Ancestral Spirits that have a color unique to them, implying they weren't tied to any Outer Gods, but were possibly the original gods of the Lands Between.
There's just so much, I think we haven't even scratched the surface yet.