I am definitely on board with more Crustaceans in my Arthropod phenotype, right now all we've got is that one weird crab looking guy and the rest are all clearly insects. I want a lobster man. And if there are enough Crustaceans, I'm not averse to splitting them off onto their own line like Arachnids did. Aquatic mammals and fish may be more difficult to do well, given any playable race needs to be believably able to craft and use tools, but they made fungus work and a snail so, I'm on board with any new phenotype or race portraits they want to add. Related note, I'd like a bush inspired plant phenotype, or Spanish Moss. I've got an idea for a Hivemind. Back on topic, I'd even drop some real world money if an entire Crustacean phenotype was as gorgeous as their plant people.
Releasing this phenotype pack at the same time as an addition of a fourth habitability type causes me no grief, and in fact would be solid marketing. Having a fourth habitability type causes me no grief either, habitable planet numbers can be tweaked so that it has no impact on the early game, and further rewarding the type of play that opens up new climates for colonization seems solid. Tweaking the random faction creator to give preference to these habitability types to spawn on aquatic type worlds I have no problem with, so long as it doesn't mess up the mix of planets. Simply adding a few prefab factions that pair the new phenotypes with the new climate would also be a great idea that wouldn't restrict play or upset balance.
Restricting these phenotypes to only having homeworlds on that fourth habitability type is too much of a departure from game mechanics to get my thumbs up. If I make lobster men, i want them to be from an Alpine world and survive in those harsh climates via a naturally produced form of anti-freeze in their digestive and lymph juices.
I'm gonna give the idea a tentative thumbs up, because I do want to play the Lobster Knights of Crustaria, a holy crusader order who reaches into the stars to right wrongs and slay dragons.