Personally would like to see next phenotype packs released in following order:
1. Amphibioids (is that even a word?) are the most obvious choice for the first pack. Toads, salamanders, caecilians etc. would make good addition to the game. Considering sheer amount of phenotypical variety among this group making distinct portraits would not be a problem. Bonus points for not requiring special mechanics: Stellaris amphibians would survive in any environment because each planet has at least some amount of water.
2. Life Aquatic pack. I'd really love to see various fish phenotypes being added in this game. In addition, mammalians should get dolphins, reptilians should get crocodiles, more stuff for molluscoids. Special mechanics are not really needed, but creatures in this pack would need their own detailed city landscape and ships to make it look like they live in some sort of aquariums.
3. Life is Strange pack with really bizarre creatures under special category. Species wearing artificial suits, low-gravity species, high-gravity ones etc. There are lots of sci-fi works to get inspiration from.
4. Earthoids pack. Nothing too crazy, just more Earth species-inspired portraits across the board. More mammalians, please, for me this group is by far the worst, least inspired and looks plain bad compared to excellent work done with other groups. More avians, more insectoids etc.
5. Silicoids pack for true sci-fi experience. There is a lot that can be done here but obvious problem is that this group may need some sort of special mechanics for immersion. That's a question for a later day, when game mechanics would take more stable form.