I already made this mechanic. This is recycled trash

The whole concept of aerial is weird.

Similar to aquatic that I've seen on Vanseeli Guard the supertype is unfortunately not backwards compatible and going forward sets you up with the tribal problem: Do you want to add the (super-/card) type to all new appropriate cards in the future?

I've explored why this is a problem with a small subset of aquatic (sea monsters) on Commune with the Depths and undead on Commune with the Dead. A batch word over a supertype may solve issues with backward and forward compatibility, but it can be hard to pick the correct characteristics anyway.

A weird issue with aerial that aquatic doesn't even have is that we already have a way to check whether a creature is usually "aerial" and that's the keyword flying.

I think an ability word for a spell ability on a permanent that still needs reminder text and looks entirely at a supertype specifically introduced just in this set is... a lot of design capital put into a mechanic that is just "this card wants to be played with other cards that are %%%". You could easily choose something like flying for %%% instead and save a lot of complexity.

And also has the advantage of making it compatible outside of set.

Still; I personally like the idea of moving vanilla keywords like flying to the supertype. Sometimes you gotta try stuff out to see if it can work.

The type line is actually more pressed for space than the text box.

How many subtypes do you think your "legendary flying creature" is going to have? :)

Both aerial and aquatic were inspired by use of the snow supertype and are intended for use sky and water worlds respectively. The supertype should only come up in planes where the supertype would have mechanical relevance, i.e. not many. This intention would be limited by your suggestion, although I do like the batch grouping for other, unrelated scenarios. Examples such as the following are why the batch grouping doesn't work in place of the supertypes (forgive the text's appearance, I never learned proper formatting breaks): Red Tide - Aquatic Creature- Germ(? may need to introduce new creature type)- When ~ ETBs other aquatic creatures get -1/-2 UEOT. 0/3. Sea Snake Aquatic Creature- Snake- Deathtouch, Whenever you cast an aquatic spell, Sea Snake gains hexproof until the end of the turn. 1/2.

Aerial was an even bigger issue, since the world itself imposes the problem of suggesting all creatures have flying. I think we would all agree just giving the flying mechanic to every creature is colorbreaking and bad for the game. The next thoughht is that every creature is airborne to a degree, so art should depict the general airborne movement of the things living on the plane. Unique, flavorful, and playable mechanics have been the tricky parts of creating a basis on which to design cards for those worlds.

As I've heard basic snow lands were a mistake, I would not include the supertype for the basic lands. Admittedly, the basic land types don't even make sense in a sky-only world (water world I think the basic land types would be easy enough to depict though). This complicates things as it forces supertype to be directly associated with either relevant mechanics or the supertype in my opinion. I have issues with some snow creatures for that reason ( why is Frostweb Spider a snow creature beyond tossing "Frost" in its name?). .

Wouldn't the basic lands in an entirely aerial world just look like on Zendikar and Drifting Meadow?

No, there is no soil or rock formations clumped together like the Zendikar lands. I really hated Zendikar's floating rocks and would be highly unlikely to ever invoke their image again. The aerial world's closest comparison I could think of would be to a gas giant planet.