I'm not a fan of adding more types to Pokemon, unless they're really ingenous and make sense (I had 0 issues with Fairy from when the speculation started), but most ideas I see suggested randomly by fans are not that good.
If however, I had to pick one that I felt was interesting enough, I would have to go with Wind. (It's nowhere near Fairy in terms of elegant execution this late in the franchise though)
The crucial part of me starting to like the idea was when I took a closer look at Tornadus' typing next to it's counterparts.
From it's reveal I've been wondering why they made it so odd and have it be primary Flying whereas the other two have an elemental type followed by Flying as secondary.
I remembered an old leak of gen4 pokemon stats, where I saw that pure type pokemon had their type wirtten twice in the coding (ie LUXRAY - ELECTRIC ELECTRIC), that made me realize Tornadus is basically Flying-type at its core as an elemental type (focused on a wind/storm-theme) but also Flying as secondary to fit the trio, it just turns it into a pure Flying type when that's the case.
I felt like they'd look more complete if there was an actual wind-type, making it Wind/Flying.
Now most peoples reaction to the concept is: "Flying already includes wind-element"
At first glance it does (Hurricane, Tornadus...), but there is a lot of things that are wind-related but have nothing to do with Flying-type, and of course the core concept of the Flying type is the act of defying gravity by some skill, hence "Flying". Like Fighting or Poison, it's a very specific technique-kind of type.
Now for it to be a good candidate for a new type, it has to do several things:
1. with 700+ pokemon existing, there have to be pokemon by now which would make sense as retcons (just as there was 1 or two lines per generation that were retconned to Fairy), with enough dievrsity to show off key aspects of the new type.
The Pokemon in the picture are some possible examples with different reasons.
-Pidgeot, because it always struck me as the special one out of the regional brids, the one I wished had a bigger Sp.Atk and focus on wind-based moves.. with Mega Pidgeot now being a thing, it seems like Gamefreak thought so too.
-Gyarados, because it honestly has nothing to do with actual flight, other than resembling japanese windsock-balloons called koinobori. That, coupled with it's often mentioned tendencies to cause powerful storms makes it a perfect candidate for a retcon.
-Jumpluff and Drifblim are examples of Pokemon that aren't so much flying but rather are affected by winds and being comfortable with it.
-Fan-Rotom: couldn't be more obvious, and it also solves the, due to Levitate, useless Ground immunity of it's secondary type, while making it stick out besides generic Electric/Flying types, like most of its forms.
-Tornadus: main reason for even considering this seriously as stated earlier.
-Lugia: it's the Diving Pokemon and spends it's life underwater, where there is essentially zero use of Flying.. honestly the only reason it even has the Flying type can only be due to a possible storm-theme, it's signature move Aeroblast and to complement other legendary birds (although it resembles them the least), most of which makes a good case for Wind-type instead.
-Aromatisse/Weezing/Vanilluxe/Butterfree are all included because they rely on spreading certain stuff in the air to affect their opponent, whether it's aromatic fairy scents, poisonous gasses, insect powders or freezing particles.
-Arcanine is featured solely for its numerous speed-related references (it's also basically called Windie in japanese)
-Shiftry is based on Tengu the wind-demon, so it had to be included (I did however have serious difficulty deciding what type to ditch, and am still not satisfied with my choice due to it evolving with the Leaf Stone...)
2. it needs to have potential for a handful of "generic" casual pokemon lines that would be pure type that further justify it's existence (just as gen 6 added Slurpuff, Aromatisse and Florges to make sure it wasn't made of 90% secondary typed pokemon), so here are my ideas I've had collecting in my head throughout the years (that I haven't gotten around turning into proper fakemon yet):
-a 3 stage cloud-based Pokemon (think, middle ground between Castform/Tornadus with a purer and simpler "wind" feel to it)
-pseudo aquatic: aquatic creature-based pokemon that are light enough to substitute aqua with sky, swimming in the air so to speak (think, flying whales)
-storm-based more abstract line (think tornado with some kind of body in the center)
-a mystic mammal that runs around on clouds (think Landorus therians feet on a lightweight eeveelution)
3. potential for moves
as the image shows, there is already quite a pool to choose from... which actually gives rise to a problem, lots of the moves are of diverse types already. A solution would be to make them all dual typed like Flying-Press, but that mechanic would need refining (would also open a lot of possibilities for other existing moves to make use of synergetic concepts: Muddy Water, elemental Punches, Earth Power, and so on)
4. add something unique to the type chart, it shouldn't be a copy paste of the Flying type with minor deviation for example, and as seen on the picture, I managed to give it extremely unique match-ups, most of which also balance the current type chart quite a bit (buffing Ice defese, buffing several weaker types defensively, nerfing OP offensive types like Ground, Fairy and Fire etc.)
Clearly I've grown fond of the idea at this point... but I'm not expecting it to turn into an official type anytime soon.