I have thought about this a lot, and honestly, I don’t think the kink/relationship aspect will ever catch on “mainstream” for the simple fact that paraphilias are overwhelmingly abstract at the best of times. Furries are incredibly easy to understand at a glance. There’s no fursuit equivalent for being big or small, sadly. It requires too much imagination, too much… explanation, as to the appeal of it, for it to take off in the same way that simple anthropomorphisation did. It’s relatively easy to explain why you wanna get fucked by a werewolf - since both the “fucked” part and the “werewolf” part are pretty well understood already.
But I do think there’s room for it to be explored differently in media. Giants and tinies have been featured across so many mediums and genres for hundreds, if not thousands, of years, but stories that attempt to take it seriously are extremely rare, insofar as none of them, to my recollection, have broken through into mainstream. I think that’s where change is most possible - in taking ideas that already exist in a lot of fiction, and adjusting the tone to be more mature, introspective, philosophical… more adult, leaning away from the traditional fantasy and sci-fi tropes (as much as I do love them). To take the idea of giants away from fairytales, or shrinking away from superheroes, and have the dynamics of size differences portrayed through a mature lens. There are examples of this already, but there’s so much room for more.
Then again, that’s not probably even the kind of content that will make macrophilia “mainstream” - I guess we’re waiting on a more populist Fifty Shades of Grey sort of thing? Which does kind of disgust me, honestly, since the mass appeal of Fifty Shades absolutely came at the cost of misrepresenting the core tenets of ethical BDSM. Hmm. Maybe we shouldn’t be encouraging this, huh…
