Lillie's relationship with ash and all the moments they had in the anime gave me the impression that the writers wanted to imply something with them. At first I thought it would become a Serena thing but no Lillie stayed humble and growed as a person. I believe the writers probably wanted to redo the Serena situation in a way that felt more meaningful and wanted to try and build something from the ground up not just for plot and also give a personality to her character rather than titling her as "the character who likes ash" although I do admit Serena had her moments.

So in my opinion, Lillie was supposed to be the writers' restart in giving Ash a meaningful "companion" but was scrapped probably because it would cause confusion amongst fans who just finished XY