I put "elemental" in scare-quotes because they aren't elements in the sense of being fundamental building blocks, even in a mystical sense. I'm trying to avoid starting with the typical earth/water/air/fire setup, and instead go with something unusual. My list is still in flux at the moment, but the basic concept revolves around different forms of energy (or lack thereof). In that sense, they are more like "properties" than elements.
Initially I was thinking of Light, Dark, Fire, Cold, Spirit, Spark, and Gravity. I want Light and Dark because I have some cool ideas for thematic Phantoms, and I want Spark so it can do extra damage to tech devices like Lightning does in Final Fantasy.
There are a couple of obvious opposites in the list: Light/Dark and Fire/Cold. I am considering splitting Spirit into Life and Death, which would provide separate classifications for both healing magic and undead-type Phantoms.
What's holding me back, though, is that I can't think of an "opposite" for Spark... apart from something like insulation/inertness, which isn't particularly exciting as an "element" in its own right. Then again, perhaps it could work if I define it as negation. Call it Silence perhaps? It could cover counterspells and warding, and maybe de-power magical crystals. I'll have to think about it.
I guess if I go with all pairs, then Gravity and Antigravity are the obvious other split. Gravity (Sink?) gets to do crushing, heaviness, high pressure, and attraction, while Antigravity (Float?) covers lift, lightness, low pressure, and repulsion.
Phantoms are magical creatures, most of which are strongly aligned to a single element and express some of its properties. A few may combine aspects of more than one element. Their elemental nature also defines the things they are resistant or vulnerable to; each will have a custom list of resistances. I don't just want to have a standard set of reliable resistances, although they will fall along thematic lines for the most part (Dark resists Dark and is vulnerable to Light).
The area around the Ark will be plagued by a few Phantom varieties, with new and unusual types encountered when the explorers reach new territories. In this way, the composition of the world itself will help maintain the sense of discovery and uncertainty as the players learn how best to handle different Phantom elements and types.
Normally Phantoms dissolve and blow away when they are killed, but it's possible to use a spell to harvest elemental crystals off them. You get different grades of crystal depending on the Threat of the creature (although I've yet to define the effects of the different grades), and there's another new spell that lets you fuse smaller crystals together to make larger ones.
Elemental crystals will have a variety of uses, including mana sources, ammunition, and changing the elemental properties of armour and weapons. I'm thinking one of the native species can also absorb the crystals to change their personal elemental alignment.
That's about as far as I've got with elements so far. I'm not sure what the subject will be next week (assuming I remember to write an article). I've been working on a card game too, so I might report on how that's going.