On the subject of gas giants, what if there is a cloud layer you can walk on?
The gas giant worlds could easily be basically more alien Aether-type biomes with giant floating jelly fish things (evolved ghasts for creativity's sake) instead of whales and clouds dominating with no islands of rock, or very few such islands.
Without a spacesuit you'd rapidly lose health on contact, not simply suffocate. The high gravity of a couple of the gas giants may also demand some new astronaut's hardware to jump and fight the pressure.
Resources I can't think of beyond what is already suggested.
Here's some atmosphere art of a gas giants to see what I mean:
As for how to vary the gas giants, here's what I thought of hazard wise;
Jupiter- constant lightning storm, more fall damage.
Saturn- Like Jupiter, but less gravity but some new hostile mob makes things more dangerous, like a flying shark.
Uranus- lightning storms do not happen as a constant unlike in Saturn or Jupiter, but instead come periodically. However, Uranus has methane clouds that either explode or get set on fire when sruck by lightning. Gravity same as earth
Neptune- by far the hardest, as it has rain of deadly crystal that rips through your health. Fun! (I suggest making shelter) gravity is slightly lower than earth's
Cloud generation patterns and objects of interest varying from gas giant to gas giant may also prove useful.
Different cloud block types would also make for higher amounts of variation.
Mob wise I decided not to think much about it since honestly I was hoping for more alien mob designs. So far the most alien mob is the slime stuff on Mars. But that's a wholly different topic.
I imagine that like with the moon and mars there would be dungeons- but the dungeon layout may be quite different due to the cloud-based nature of the gas giants. Like floating temples or something.
Things can get even harsher interstellar wise (there are known exoplanets that rain liquid metal for instance), but I believe if anything interstellar is done chances are it will be done in the form of a mystcraft-like world randomizer.