I view this as kind of a non-issue personally. If you're concerned, some thematic expansions (both within and unmentioned by the rules) can include
Water: Blood, Acid, Kinetic Energy. Just generally liquid and motion are major themes.
Wood: Poison, "Chi," animals. Not to mention the wide ranges you get from plant life in creation.
Earth: metal, Sonic waves. And they have that whole focal crystal/awareness thing going on.
Fire: Light, Shockwaves. And c'mon, it's bleeping fire!

And I disagree about lightning not really belonging with air. Meterological effects and all, and one without an obvious other element to group with (rain goes with water as an obvious example).