The traditional four elements are Air, Fire, Water, and Earth. I've always felt like this wasn't the optimal way of dividing things for a lot of little reasons. Fire, as our modern minds know, is a process rather than a substance. Air, Fire, and Water also all share a lot of traits; which one is the element representing change? Would that be fire, which is literally a change? Water, with its ever-shifting currents? Wind, which is formless to begin with?
To this end, I propose rolling fire into air. (I'm calling it Aether for right now until I think of something better.) They're both very fluffy, and only having three primary elements just makes everything a lot simpler. Plus, as an unexpected bonus, it now tracks nicely with the human experience: Aether is spirit/emotion/willpower, Water is mind/knowledge, and Earth is the body. The energy types exist within these or between them.
While we're here, let's throw out "Arcane" and "Force" as energy types. Spells can just exist as the elements we've been talking about if it matters. If it shoots magic bullets at something, those deal physical damage just like arrows. Healing is air magic, since it's primarily a spirit thing. Teleportation goes under water because water has a strong narrative connection with travel. Etc. Again, when we get closer to time for me to run this campaign I'll look into setting the nitty-gritty details.
Elementals should not have any kind of pseudo-ecology. They're like diamonds, formed from extreme concentrations of energy on other planes. Actually, earth elementals might BE diamonds, who knows? That seems a little gimmicky. Scratch that. So in order to acquire and bind an elemental, a wizard has to first locate one within some great (frequently extraplanar) maelstorm. They're thus very rare and very powerful, because little elementals are boring and work to trivialize magic. Also, because I believe in consolidation, Aether elementals are djinn, Earth elementals are True Golems, and Water Elementals don't track well onto anything in particular. Water is associated with knowledge and travel right now though, so their role tracks very well with the kind of things that a wizard has a powerful extraplanar servant for. They scry, they teleport, and they whisper secrets. (Perhaps, as the most cunning of the elementals, they demand a "price" for their services.) I like the image of the magic mirror with the face protruding from the surface and leaning down towards you. "Can I carry you from this place? Boy, all the world's one river."
Dragons! My nitpick with dragons is that they take too long, and move through too many stages. It takes a hundred years for a dragon to reach horse size, and two hundred after that to outweigh a gelatinous cube. So my solution is that they can hatch at any point of their development. A dragon egg, left undisturbed, sits quietly and absorbs energy. The type absorbed determines the type of dragon. Thus, eggs left in a volcano make red dragons, and eggs acquired by the church hatch into gold dragons, as they soak up righteousness. Wizards love dragon eggs for this property; they're the thaumaturgical equivalent of the glass of water you dip your paintbrush in, if paintwater had the potential to destroy you utterly. Wizards can also harvest the stored energy, and dragon eggs are the most "common" ritual component, to the degree that anything involved in epic level magic can be called common. Most wizards do not have a dragon's egg, but amongst the handful of 14th level wizards, maybe half of them do, with about half of those having two or more.
Once a dragon hatches, it is first and foremost naturally charismatic. Dragons are the natural lords of the Material plane, and everyone knows it on some level. (This does not mean that they have mind control; mortals tend to respond to draconic authority in the same way they do to mortal authority, albiet often more dramatically. People with an anti-authority streak hate dragons.) When they signed the Big Pact of Light and Dark that every setting has, dragons were the signatories representing the Material plane. Some dragons can shapeshift, and prefer to interact with human society without revealing themselves. (this is the origin of some of the world's legendaries heroes, who appeared from nowhere, amassed a large following almost instantly, and then vanished, promising to return when the time was right.)
Stay tuned next time for Planes, Undead!