Classical Element Combinations

Please note the following isn't a structured argument for any one system but a mad man typing down thoughts as they come to him. This author may change his mind mid post and contradict himself in places. Please read accordingly I actually really like the pokemon system. This basically means you'd combine the two weaknesses and strengths into one. Although that does create some funky weaknesses like you mentioned...

Another way to do it is that you give each element a priority. Thus Fire could be the top and then go down from that. Which ever element is lower on the list will be the element you use to determine weakness, while the element on top is the one you use for strengths.

This does present a problem though in that this inherently means you'll have one element that is always better in a combination than others.

Another idea comes from the Pokemon TCG. This is that the weaknesses are right there on the cards. You could do the same with the different elementals. You could have the generic weaknesses and strengths, but then when there are combinations, you might want different strengths and weaknesses depending on the card. This means that when creating different elementals you actually have a greater power in customizing each card now! You could even start classifying their attacks so that a steam elemental that IS steam, it's attack could be water, but it's not weak against water, or water isn't good against it but perhaps Aether is for some odd reason. Either way, this gives you more flexing room.

Ah, you want a full circuit then. Well then. I don't really know a solution, unless you made metal good against dust good against mud, which makes even less sense, not the metal vs. dust, that makes sense in that dust can't harm metal, but dust being good against mud.

You can also think like this:
If an element is weak against another element. Like Air against Fire. Air can actually be used to fuel that other element.

And Steam fuels Electricity. A cloud is steam. A cloud can produce lightning. And air helps the steam with it. Thus is sounds indeed logical to me too that air beats steam.

Overall, it depends on the area that you are targeting with your game. While the west has 4 elements as basic. The east has 5. And there are 2 versions in those 5. One has lighting added as the 5th element. And the other one contains wood and metal.

Wait, have we been down this road before?

Strong and weaknesses of lava. Well, lets think logically about it. The most important thing about lava is the temperature.

In real life the following effects of elements take place when facing lava:

Water vaporizes, but cools lava. Eventually water will win.
Water is the weaker on in this, but abundant. It is often a quantity against quality.
Besides, the outer shell of lava cools first, trapping the rest.

Earth, eventually melts in lava. But has also a cooling effect on lava in the same way. If a bit of earth melts and immediately cools, the same trapping takes place.

Wind has no effects at all. And only gets heated and gets energy from this to blow around harder.

Fire, ehm, actually gets produced by lava.

Wood, burns and has simply no effect on lava.

Metal, eventually starts melting. You can use lava and fire to get metal in the shape you want. Thus making useful tools. Like an axe to shop wood. A bucket to carry water or earth.
Although, metal guides energy much better then water or earth. Thus metal cannot cool down lava, but becomes part of it. Making the lava stronger. If the lava is not cool enough, you simply have more lava.
Perhaps you can make it so that you can expand lava by adding earth or metal, things that only melt. And if you add enough, the lava is too cool and becomes earth.

If I think about aether. I don't know this one. I can't get a grasp of how it looks. But one thing is sure, aether or void are the opposite of dark holes. Dark holes are called this way because they look like holes. But they are actually the most dense objects in the universe.

Void = Vacuum?
In that case, void would be the best medium for light to go through. There is no resistance.

I guess, this is the trouble you get when using aether.
But don't give up. You could try to see how other people apply aether in their games.

There is a minecraft mod using aether, that I happen to know of. See what they have added on the wiki? And do your experience.

Edit:
I was just wondering, what is your current list?
And I am brainstorming on this right now, using a different approach.

And I don't know if I mentioned it before but each element can have 2 properties cancelling other properties out:
Earth: cold, dry
Water: cold and wet
Air: hot and wet
Fire: hot and dry

dry and wet cancel each other out.
cold and hot cancel each other out.

Lets add fire to earth.
cold+hot+dry+dry= 2 x dry; sand
lets add another fire
2 x dry + hot + dry = 3 x dry + hot; lava

When we are going to fight lava with water we get:
3 x dry + hot + (cold + wet) = 2 x dry, we get sand again.

Now, this might still give issue's regarding stuff like ice. So I think, Aether's only job would be strengthening one property. While maintaining another.

When we combine earth with water, we get 2 x cold. Thus mud is 2 x cold.
Yet if we apply aether to water, we get ice. which can be 2 x cold with wet or cold + 2 x wet.
Should we have 2 different ice? Well, we can have snow besides of ice.

I also see an issue with water + air, which gives 2 x wet.
Well, that makes no sense to steam. Right?

You could try using the elements from the avatar.
Where each element has an advanced version of bending.

Fire, Lightning
Earth, Metal
Water, Blood
Air, ??? still to be told by the creators.

There are also other advanced versions like healing with water.

But Aether could be the bridge between the classical element and the advanced element. You already have your metal right.
But your Fire + Air = Lightning is a flaw either way.
Just my thought.

My suggestion was that Aether makes the elements more "advanced". But you could call it a combination. I don't see the difference except for the naming.

***

Regarding Lightning:
From a physics kind of guy point of view. It is totally flawed ;).
Wiki!!!

In ancient times they didn't know better. But the classical believing was indeed:
fire + air = lightning
Or
lightning is a separate element
Or
lightning is an advanced version of fire.

If it works for you, go with it.