Overall I liked the variety of the missions. The only mission that I didn't particularly care for is #5 (Unsuspecting Allies). The cultural objective felt more tediuous than the other objectives in my opinion. That being said it did provide variety of objectives which was good.
A balance of alliance, elimination, and capture the planet missions would be a good start. In addition, I'd like to even more variety of the type of objectives mixed in.
For example, find and destroy a unique space creature before your opponent does, need to capture a number of planets that each contain a part of a larger device, steal or capture certain special technologies available only to your opponent, destroy a certain planet with a terror star, find and destroy super terror star that opponent has,etc. I think you get the idea.
I also think the more objectives available for the campaign and scenario editors would increase the life of GCII significantly.