Number 1: Expedition Leaders
Buy this firstAdds: Six leaders, each with their own powers and starting cards. Also a new research track, plus new items, artifacts, sites and guardians.
Fixes: Every player starting the same way. Leaders give each person a different plan from turn one.
The go-to first expansion if you only own the base game. Adds variety without changing how the game plays.
Usually a better fit for a different kind of group — still fine if it sounds fun.