I have done mythical creatures and land animals as well as birds so far, so I figured I should do one more list of plants and water based pokemon. Depending on where the map goes, we might not have much salt water based pokemon, so if I include any, it won't be many.
Pike - This would be the fish pokemon of the generation, Sometimes we will get 2, however we have been lacking in fish pokemon for a few generations now.
Football fish - Although they normally don't live in colder water, I think this could be a great fish pokemon given the fact that it is called a football fish.
Greenland Shark - This pokemon wouldn't be a stand alone, but it would go perfect with the Galar Region, it could be a cold shark instead of a dark like SHarpedo, so maybe Ice/Steel or something. I guess the Pike could be the pre evolution of this pokemon.
Narwhal - Another good cold water artic life.
Beluga - Because LGPE had the code name Beluga, I wouldn't be surprised if we get a Beluga in these games.
I tried to avoid seals this time around, as I think we have plenty. Walrus is the same way, but I think we could afford to get another one. Now on to plants. Honestly almost any plant could be used, so I am going to focus on native planks. Even if they are native, it seems unlikely that they are going back to flower pokemon. As we haven't had any. They missed the perfect chance in Alola to make a Hibiscus pokemon which they didn't, So not gonna make this list as big at the artic water pokemon.
Snowdrop
Primrose
Mistletoe
Honeysuckle
I would also love to see another line based on a fruit similar to Cherubi and Bounsweet. I think an Apricot pokemon would be a great idea for a line. As well as a berry pokemon. I'd love to see a pokemon that looks like a berry or has a ton of berries growing on it similar to Tropius.
Gingerbread I think could also make a great pokemon if done correctly. I know it isn't a plant, but since the Galar Region is cold, I am also grabbing ideas that usually show up around the holidays.