Ooo... The atmospheric planets. I do want some, but I imagine it's quite hard to do them in a game like Elite. I am a person with rather limited scientific knowledge when comes to physics, chemistry or geology so please do forgive me for, what most certainly will be a very simplistic view, what atmospheric planets should entail.
Atmosphere impacts how our ship will behave during a flight. And that depends on the density and composition of the atmosphere which varies greatly from planet to planet, so you can't just flip a switch and boom, you have altered flight model. Each ship will fly differently depending on it's shape, so new variables will have to be added to every ship, and those variables, combined with composition and density of the planet, will determine how the ship behaves (and that's on top of gravity).
And since we have atmosphere, we will have wind as well. Another variable that impacts on the flight model. And please remember that wind is not constant, it changes all the time, it has different strength and direction depending on the height.
Wind also means erosion, and that means completely different way of generating ground. Right now it's "relatively easy", but with wind you have to take into account that different materials are affected in a different way by the wind. Some will erode really quickly while others will stay solid for much longer, and that will result in entirely new geographic features.
Wind also means floating dust or sand which, in strong winds, would mean sand/dust storms.
Storms mean weather that will have to be localised and dynamic (again affecting flight model), which will mean clouds.
It's not inconceivable to expect some sort of rain from said clouds (not necessarily water), and that gives new ways of erosion and therefore ground generation.
And since we have atmosphere with variable density, composition and weather, that would affect light on the planets as well. Light that passes through the atmosphere is dispersed and may change colour, and that has to be simulated by the game as well. Even on Earth the sun light is white-ish in midday but turns red by the evening or morning because the light has to pass through more atmosphere. And since we will have different colours in the game thanks to various stars and variable atmospheres, the variation in colour could be massive.
...
I'm probably missing quite a lot here and I'm not even touching here on planets with life, but I guess that if you sort out the above, adding flora should be relatively straightforward. Might be a bit messy with fauna (with pathfinding, AI, behaviour, pray/predator types, damage dealt/received, animations etc), but that's a different story.
Now... Of course FD could implement the above in stages, but I am willing to bet my T6 (I really like my T6) that within 10 minutes of staged atmospheric planets going live, this forum will be swarmed with threads like: "Why doesn't this include X, Y or Z?! This sux, you had ONE JOB Frontier!"
Oh well.