1. Yes. 100% planned, but not in the next major update / RWG. The weather model was designed specifically from the grounds up to be able to model Mars, Earth, Titan and Venus (but a small amount of the higher temperature logic remains to be implemented). Venus is a logical terraforming target, but going there from the start does not seem possible judging by its ridiculously high temperature. Electronics would likely just fry with current technology. Some sort of advancement needs to be made.
2. Yes, stars will have different classes/size. I believe some story stars will be based on real known stars, but certainly any extra-solar world will be mostly made up.
3. Absolutely, and available even as part of the RWG/next update, with some realistic cap. I expect the ones in the next update to be somewhat easier than smaller world due to the way the column mass of gases relate to pressure. Later on though... we can imagine there are some worlds, that are very, very, very large.
4-5. :) Not answering those two. Some surprises have to remain surprises. I will just say I am well aware of these two ideas :) I will also say Dyson Sphere is definitely planned to be in the game, but not as a terraformable target.