I say this a lot, and I'm sure my ID changed, but my all-time inflation fantasy is that someone invents a safe, proper way to do air-filled, adjustable implants that can inflate and deflate freely.
Not even entirely a fetish thing, it's just an interesting idea, women being able to go about their lives normally, no issues, no complaints, having smallish boobs until they go out for a party or something and give themselves a boost, going up a cup size or two, then deflating back down after the party. Air, water, helium, whatever.
Bonus points if someone can come up with an inflatable belly implant, but that's more of a niche fetish sort of thing, so I don't see it happening.