Heya
There are 3 types of Pokemon types imo: elements (including subjects of nature like rock, ice), mystical creatures (dragon, fairy, ghosts) and abilities (to fight, to use psychic powers, to fly)
I have a similar system actually (that I've changed like 4 times by now, this is the one I've been satisfied with for the longest time):
Species-types in mine are types that can relate to what kind of creature the Pokémon is, or can be worn as an armor.
And I see Ground foremost as one of the "abilities", like a counterpart to Flying. Not about literal earth, but just the concept of a firm ground that you can dig and disturb to cause damage due to the fact that your opponent is likely stuck on the ground as well.
That viewpoint also makes Ground and Rock more distinct, which helps a lot as an analogy for Flying VS Wind, where the former is also an ability, while the latter is more the actual element.
Demon
Troll
Serpent
Alien
Angel
Dance
Healer
Digital?
Light
Gas
Troll/Demon - could you find some examples out of the existing 802 Pokémon that would somewhat represent the essence of these types? I'm having a hard time thinking of any. Demon specifically sounds to me like it would be what Ghost is (if you just ignore the name ghost), morbid shady creatures... which reminds me about Sableye, probably the closest to a candidate fro Demon? Perhaps the Deino line swapping out the Dark part?
Serpent - while I can see some of the snake-like Pokémon (Arbok/Serperior/Milotic) as easy implementation, I also imagine it would clash with Dragon, since Dragon is a "wider" mythical creature type that encompasses seprents to some degree. I guess my point is, Dragon is already one of the less represented types, and getting any Pokémon that would qualify for a Serpent type (which would be an even smaller group) would still not make Dragon more than uncommon. An analogy I can think of is if there was an arachnid type added (which would just take the few spiders we have from Bug, and possibly include some crabs and scorpions, and it would still be a bigger group than Serpent). One dilemma case I see for retconning would be Onix/Steelix.
Further type ideas along those lines that "Serpent" made me think of are: Slug, Lizard/Reptile, Mammal, Dino, Beast..
Alien - I have something coming up in the next post that is relevant to this one
Angel - good and not necessarily ethereal spirits.. sounds very close to Fairy. It's just missing the religious connotation, but I'm not even sure whether that would be a good thing or not.
Dance - I was actually thinking it would make sense if anything dance-related would be covered by Sound. I mean, types often include themes that their names don't directly imply, but otherwise naturally synergize with. Think of the Sound type as the "Music" type (notice I have Bellossom and Maractus as Sound retcons for example). This does remind me that I forgot to mention a "dilemma"-case for the Sound type. I couldn't find a way to make Ludicolo part Sound. It's interesting actually, because just like with Shiftry, it's the evolution method that's preventing me from making Ludicolo Sound/Grass (it evolves via Water Stone).
Healer - I guess if something as specific as "Poison" is valid, then you have an interesting point there. I can't think of more than 3 families as candidates though (Chansey, Audino, Alomomola...perhaps Meganium?). How about calling it a Life type? Also, I was a bit surprised that Fairy didn't grab anything and everything that such a type would entail for itself (Chansey and co aren't actual Fairy types and the moves relevant are either Psy or Normal), but I would propose to merge whatever a hypothetical Healer type would encompass with the Fairy type.
Digital - makes you immediately think of Porygon.. but can much else be done with it beyond that one line?
Light - not in the sense of goodness and life, but rather as pure elemental light, right? I usually dismiss it simply because any examples are probably already Electric and happy with that. I was going to browse what Pokémon learn some generic light-moves, but realized there really aren't any such moves to begin with. Flash isn't learned by anything noteworthy.. so I'm curious what the list of candidates would be besides Ampharos and Lanturn.
Gas - I was thinking of naming the Wind type an Air type instead, but since I decided to also have it include focus on speed as one of the aspects, Wind felt more suitable. It's also more broad in things you can spread through the air for indirect damage (spores, ice particles, heat, sand, and poison or smoke of course).
As for Fish... I went down that route as well, will make a post about my take on that one later.