by contiguity
There's a fair amount to explore even in the base set. For example, knowing when to do military, colonize, or both, with each of the original worlds. (Fertile makes colonize easier, but also gives early access to Improved Warfare with a colonize symbol. Metallic allows you to choose improved colonize early, then switch to warfare soon after. Advanced can be a mixture of these.)I don't feel like it needs additions, but some of the new planets are fairly interesting, as well as a few of the new technologies.