Been having fun with the retcon aspect that (with any halfway valid type) should be unavoidable in a "what if type X was added" scenario. Initially I was just contemplating one type idea at a time, but I figured that I've narrowed each of them down to distinct enough concepts that there wasn't any serious overlap between them, hence I decided to consider them all at once as well.

pic related is all currently existing PKMN with their types and my retcons of them.
(pic with only retcons coming in reply)


Some thoughts in regard to each speculative type:
-Aquatic
The main point of making this one was to split up the most common Water type (133 out of 802, in other words, every 6th PKMN is a Water type).
It sounds super silly and redundant at first, but hear me out.
Ever since the Tynamo-line (along with Stunfisk) got introduced, I've realized that being an aquatic creature does not have to equal being a Water type, and have hoped that we'd see more and more examples of non Water type fish and other aquatic creatures (Malamar is another line that comes to mind, but one could also count in some of the fossil-PKMN as well as ones like Crabrawler). The Tynamo-line is also the perfect example because it doesn't learn practically any Water moves.
Simialrly, the opposite holds true as well. Being a Water-type does not have to equal to being an aquatic creature. Meaning you can have Water PKMN that have Water slapped onto them as an element, like for example Suicune, Keldeo or Simipour.
So Aquatic as a type would on the defensive side be akin to some of the "species" types (Bug, Fairy, Ghost, Dragon..), including most fishes and sea-dwelling creatures, and on the offensive side it would be like a "skill" type (Flying, Fighting, Psy, Poison..), which would encompass being able to battle well in water (Aqua Jet) and general things like swimming up Waterfalls, Diving, or using body-parts more common in certain aquatic creatures (Crabhammer).