Perhaps the real limitation of Gas Giants as planet type is the size. We could have already Gas planet, just not "giant".
Even that's not much of a reasonable point anymore. I've been playing games with size 1000 planets every now and again with fairly little lagg (until late game, of course. Unit instancing pls.)
No, there really shouldn't be any size limitation. The limiting factor for planet "size" isn't really size at all, it's detail. For the planets we have now, the larger the planet, the higher the polycount, the more flowfield data... a lot of things scale with it. With gas giants, you don't need a very high polycount. The planet only has to be round, it doesn't need brushes or features, it doesn't have to have the same level of detail as a rocky planet. Your gas giant could be much larger than any regular planets, have the polycount of a scale 250 planet and would probably still look fine.
Not to mention that jupiter is actually small, 80% of the planets we discovered in other solar systems are gas 'giants', half of them 10x bigger than zeupiter. Also it's very strange to see saturn without pretty rings... Honestly there shouldn't be a planet size limit in this game, your computer is your only limit. Watch us play PA over 10 years on size 6000 planets with 20 players each.
To be fair, a lot of this probably has to do with our methods for detecting extrasolar planets. Really big planets are way easier to detect than smaller ones, so we happen to find more of them proportionally.
This is true, it was only within the last decade that we could even start detecting rocky planets at all...
If the model was the bottleneck (it's really not, but...) you could even go a step further. With the right shaders and a high res animated texture you could use a 2 poly plane (a square) use the "face camera" flag and you could scale it to any size. This is a model I made a few years back. The effect in the center uses the technique described above laid over a sphere model. It was very low poly and worked well in an RTS.
