Because basically anything we could ever want has been added because it's been four years.
Not really, at least if you consider more than 10% of the vanilla options worthwhile (so yes, if you only care about mechanical functions you're mostly correct).
Aesthetically we're missing a lot of arthropods e.g. bees, butterflies/moths, scorpions, crabs and so on. Not sure whether worms or normally immobile animals would be viable, we do have snakes and land-octopi though. Plants and dinosaurs are a bit "out there" but I see no strict reason not to have them.
Mechanically better support for mixed creatures e.g. chimeras, manticores, sphinxes and the like would be nice. Gryphons and the way centaurs currently work would probably fall under that mantle too. And I am not sure to what extent the current mechanics support seahorse pregnancy, egg incubation effects do exist suggesting males could at least carry them orally or anally.