I'm convinced that the room temp superconductor is real for a couple reasons- the academic infighting around who gets credit and the nature of the evidence (more can be read here: https://en.wikipedia.org/wiki/LK-99). Superconductors allow magnets to float on them via the Meissner effect- if we scale this up in time, we can have a floating city before 2050. I'll define "floating city" as a settlement that levitates 10 feet off the ground and houses 450 people.
Is there a requirement that it's levitated by RTSC, or are we including other technologies like lighter than air platforms?
Aerographite structures could well be a route to this (ultra light, could be passively levitated on thermal currents). It's a challenge to synthesize that much of it, of course (current technique is to deposit carbon on a zinc oxide tetrapod and then etch this using H2 and heat).