I'd like to see gas giants added to the game.
The primary reason to add gas giants is for sake of the aesthetic appeal: real gas giants are beautiful, and there are countless artistic renderings of imagined gas giants. If the upper atmosphere of the gas giants were navigable, that would also make a visually interesting environment.
Gas giants are primarily composed of hydrogen gas; while hydrogen isn't that hard to come by, it could be particularly easy to harvest hydrogen from a gas giant. Imagine ships "skimming" gas giants for fuel, for instance.
Gas giants could also present a hazard, with particularly strong gravity wells. While they wouldn't have a surface per se, a ship that descends too far would not be able to escape.
They need not literally be larger than other planets, or at least not significantly larger.