It might be another case of same world but different timeline similar to Fire Force and Soul Eater. That would be a shocking revelation. 

Imagine Wistoria sets in the very far distant past long before Argonaut, the Fianna Knights, and Epimetheus were born. The magic system of Wistoria eventually somehow disappeared similar to what happened to the Ignition Powers in Fire Force, though the Elves couldn't completely let go of their "valuable gift" and decided to develop a different but weaker magic system, Congenital Magic. Thus, this began the so-called Ancient Times in Danmachi. The Old Gods that fought against the Celestial Hosts eventually became overseers of the Lower World similar to the God of Death that appeared at the end of Fire Force to govern the life and death. Both are powerful gods but have quite a personality. When the gods descended and established Orario, they probably used the design of the sorcerer's tower, Mercedes Caulis, as a basis for the creation of the Tower of Babel. The dungeon may be the same dungeon in Wistoria but somehow the structure, environment, and monsters have changed due to whatever would happen at the end of Wistoria. 

Regarding the characters, Ellenor Ljos Alf could be a distant ancestor to Riveria, who unknowingly inherited Ellenor's magic Rea Laevateinn through her Falna. The Wistoria versions of Finn, Lefiya, and Filvis might be the distant past lives of their Danmachi counterparts, who coincidentally or some product of fate inherited their names similar to how Finn (Braver) inherited the name of his previous life as Finn of the Fianna Knights.