Dumb theory ahead: what if Rosaria is the former Queen of Lothric?
1. The Queen was considered to be like the goddess of fertility, Gwynevere; Rosaria is a goddess of rebirth, and her soul unlocks a miracle of Gwynevere
2. The Queen's "holy maiden," Gertrude, was a conduit for Angels; if one examines closely, the Man-Grubs appear quite similar to the larval beings "commanding" the Angels in the Dreg Heap (are Man-Grubs pupating Angels? or are they merely Apostles?)
3. Angelic Faith seems to play a crucial role in Lothric and its surrounding geography, and it's possible that Sulyvahn, Aldrich, and the Deacons of the Deep have some fascination with the beings, judging by certain statues and design details and similarities to the "Pilgrim Butterflies," also seemingly connected to the Angels
4. The Queen apparently went mad after the birth of Ocelotte and "quietly disappeared;" Rosaria also had an issue with childbirth, when her firstborn broke her heart by "stealing her tongue" (Lorian?)
Alright, it's kinda dumb, but it's a fun idea to toss around.