But point is I don’t think you have to think about how the players will exist and live on these planes and how to populate them with features as much as you can just focus on one place in the plane and that’s where the PCs end up, a la the City of Brass in the Plane of Fire. Maybe the Plane of Air is the Sky Tower of the Wind Dukes of Aaqa, or a graveyard of lost ships from across all the multiverse floating in the Plane of Water.
Alternatively, you can think a
lot about how players can live and exist and live on these planes. Like, the plane of air, are you just falling constantly? Is there a terminal velocity, or are you just accelerating forever until you eventually break up and or get ejected into a neighboring quasi/para-elemental plane? There likely isn't a gravitational "center," or it would have long since become a plane of not-air from the accumulation, but perhaps there are local centers of gravity that shift and change, and part of the challenge is predicting where safe zones are and how long they are likely to remain before breaking up and dispersing to newly created accumulation points.
The plane of water is probably the most survivable since there is already magic to deal with the breathing problem, but again, you have to wonder about how atmospheric pressure works in an infinite plane of water. Is it crushing depth everywhere? Does the pressure change depending on where you are in the plane? Do ocean-like currents exist in a plane without wind? As EzekielRaiden mentioned, the food chain, if it exists, is likely to be completely different from what we would expect in waters on the prime due to a lack of a real "bottom," but the waste from native life has to go
somewhere, so there would still likely be some predictable sort of pattern to life there.
Fire is an interesting one because even fire-protection magic isn't really sufficient to deal with a plane where it's not so much that everything is
on fire as it is that everything
is fire. Water is instantly evaporated. Air is instantly consumed. The same goes for food. Existing as a mortal being just isn't possible without extreme measures. The best approach would be to take on an elemental form of some sort, and
then you can start worrying about how physics work in a plane where fire burns without a source because it is the source of all fire.