It'd be most appropriate for a leech pokemon to learn moves that heal allies :D
A swordfish seems like a strong candidate for water/steel. I had an idea for one where the sword would be chainsaw-like.
I've always wanted a virus pokemon. I don't mean a legendary humanoid alien who happens to be made of "space virus" or whatever, just a
virus, like this T4 Bacteriophage:
Some other microbes would be cool too, like a diatom, the spaceship-looking algae making up most phytoplankton:
A good old fashioned bacteria:
Or a dinoflagellate (some species cause the toxic "red tide" phenomenon) :
I thought it'd be cool if there were microscopic pokemon which you collected as
held items, like parasites/symbionts on wild pokes, and then, like fossils, took them to a laboratory where they were enlarged into "real" pokemon.
Oh yeah, I'd also like to see a cocoon/chrysalis poke one of these days with a split evolution into either a butterfly/moth and a wasp or fly, as certain wasps and certain flies are parasites who lay their eggs in caterpillars, replacing the moth/butterfly during pupation.