It's my belief that Salem's endgame with the relics is to try and revive her children. Given that we still see her acting motherly even towards her subbordinates in the opening of Volume 4, and we know that humanity was made out of the concepts that the relics are empowered with, I like to believe that all Salem wants anymore is her children.
Could even imagine a small scene in later volumes of Salem entering a magically projected room in the likeness of her little girls' bedroom. Sheets somewhat disheveled, toys lying about. Salem, on motherly instinct, puts the toys away in their chest, meticulously makes the beds and flffs their pillow, her face invisible to the viewer all throughout. Then she just collapses over one of the beds, weeping quietly as the illusion fades away, and the emptiness of a barren stone room is revealed, Salem's sobs echoing.
Would, of course, ultimately result in Salem's kids all probably getting Thanos'd in her arms by the GoL the same way Ozma was. But it would be pretty great to see, if morbid. ^^;
tl;dr Creation, Destruction, Knowledge & Choice made humanity, Salem wants the relics in hopes to resurrect her kids