Earth: Abundance
Buy this firstBGG 8.06 from 838 ratings. A BGG review calls it a smooth fit that is easy to leave in; a poster says most of their players choose it for the seed abilities and sprout storage.
Needs: Just the base game
Adds: A sixth player, a seed resource with a Germinate action to search the deck, 80 new cards and an extra board for storing sprouts.
Fixes: Seeds help you find the cards you need for fauna goals, and the extra board lets you store sprouts you cannot place yet.