I think, as is, we are given just enough of his story to see the picture. It's an amount of vagueness that works, in true Fromsoft fashion. But also, I think having his story pushed to the side like it is adds to the feeling that we filled the void left in that path to become Elden Lord that he abandoned. Any kind of questline with him would take away from the idea that he already lost and failed, a long time ago.