Elementals are boring. A clear framing of the problem and some good solutions for the basics of the problem are here on Chris Hogan's blog, but the underlying problem--which generates the others--is they lack personality.
To take a 3000-foot view, there's just too many conflicting conceptual demands on them--on the one hand, earth air fire and water are supposed to be interesting because together compose everything. On the other hand, the metaphysics of D&D (and Shadowrun and RuneQuest and Elric and RIFTs and other games that use them...) have other Primary Forces with way more personality like war gods and Tiamat and Cthulhu. The dullness of their powers and appearance and behavior comes out of the fact they don't really have a thing.
Taking a tip, like yesterday, from a dead Milesian philosopher--this time Anaximander, here's one: the elements are always fighting each other. Every time you summon an elemental, things go out of whack. They live on their planes, but they have reluctantly agreed to let ours be neutral ground with no one element predominating. Magically bringing a sentient quantity of any element onto our plane fucks that up.
You summon a fire elemental, things go ok for 2-3 rounds, then the earth begins to rebel, the water and air, too. You can make a table for each element, like tremors start, the moisture in your body begins to increase and slow you down, the wind picks up or it gets hard to breathe.
Any elemental monster on our plane will eventually attract three counterparts to tear it apart. Water weirds bring xorns and dust devils and firesnakes.
Rather than being personified like scheming gods, the elemental consciousness is chemical--they just seek a balance. It may be too much of a stretch to imagine them as complex villains while keeping to their extraordinarily simple (and thus easy to grasp) themes, but making them always and inexorably react to each other at least means each one implies something else--and that's a big step toward making them move from just a ball of numbers wizards whip out once in a while to adventure fuel.