Likely the first or one of the first will be the aquatic expansion, letting you create aquatic creatures and undersea cities and such. He also talked about a fauna expansion, which honestly I think would be hella boring.
I say go with the aquatic one first, then do ones that expand the gameplay of each phase. For example one that makes the Tribal phase fun, then one that makes the Civ stage fun, then one that makes the creature stage the living breathing ecosystem we saw in early gameplay videos with all the creatures intermingling and such with herds and solitary hunters or pack hunters etc. instead of all the creatures pretty much staying put at their nests.