I know we have enough bugs but more unique insect/arachnids would be cool. Some ideas I had where Roach, Scarab, Stick-bug, Wood Louse (I think its a crustacean actually)/AKA pill bug, Centipede/Millipede, Tarantula (ariados is more a common spider), Ant (trapinch could get a split evo, but I was thinking something more like bug/fire or bug/ground and have a "queen" evolved form), Mosquito, Horsefly, earwig, Weevil, Bombardier beetle (really surprised they haven't done this yet), and Tick or flea. We have enough Moths/butterflies.
There are many prehistoric animals that would make good pokemon. Dinosaurs obviously are the most diverse, but would end up being rock since they are fossils. A Raptor or some kind of Therapod that was Dragon/Rock would be awesome. Tyranitar is more Godzilla based, Sceptile is a Gecko, and Rampardos is a Pachycephalosaurus..which isn't a therapod..so yeah it hasn't really been done yet.
Terror Birds are also really cool. A Bird of Paradise would be awesome. Cappibarra would be neat. Cassowary would be cool. Koala bear is needed. a regular panda and not a drunk one is needed..Maybe Spinda can evovle..cause it needs too. A wolverine would also be really neat.
No more fish.. we have enough.