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I actually prefer the SM romance to the XY one.
Think about it. XY was really forced. Literally: "Here's Serena. She loves Ash."
And that was it. Right from the getgo they just retconned Ash's past to include some bit about Serena to give her a raison d'être.
With Lillie it's different. It'll be the more interesting type of romance. The type that develops over time.
From being a shy noble girl that can't even touch pokémon to being a strong trainer herself that'll have the courage to stand tall and face her own evil mother to save pokémon, she'll really undergo some changes.
And all of that with Ash at her side. He'll be there to protect her, comfort her and enable her to grow as a person and trainer.
Naturally when two people like these spend so much time together (which they will once they start traveling and she gets her ponytail[another symbol of her undergoing changes and progressing]) it's only to be expected that some feelings arise. Lillie will definitely fall for Ash and see him in a romantic way as well. They'll bond and grow so much closer.
And I find this to be a really intriguing romance, one I'm hyped for.