Are there any plans for Terragen to be able to produce a global cloud system like this?
If you mean a full planet atmosphere fluid dynamics simulation, then no. If you mean something that *looks* reasonably like that, you can already basically do so. It takes a lot of fine-tuning, but the built-in procedurals and warp functions can create pretty realistic global cloud cover. Take a look at some of NVseal's work, or Freelancah's. This is entirely procedural: http://nwdanet.com/buy/7-preset-packs/51-nvseal-global-clouds-pack and with a bit more warping, you'd see more vortex-like stuff as in the cloud map you posted. It's not a proper simulation, but you can get surprisingly good results.
- Oshyan
I saw quite a bit of improvement after just pumping up the lead-in scale (i.e. noise octaves) for the Cloud Density Fractal.