Maybe you could have, like, the "Temperature" tree which deals with heat and cold, which is more the pure magic damage tree, and an Earth and Air tree (Paganism? Or more like manipulation of "Substance") which is more utility oriented with buffs, terrain manipulation, knockdowns, etc.
The problem with such approach is that you'd still end up with what's essentially a tree for fire, water, etc. which results in either overtly broad set of effects each tree encompasses, or a mandatory 2-tree-per-skill setup, where "fire" alone and "support" alone do next to nothing and only together they form "Burn My Eyes" or "Haste".
Trees being themed by functionality, rather than just themed by "theming", allows for a tree to contain a vaguely complete and functional(although somewhat narrow) skillset, while avoiding the aforementioned trappings of being overtly broad or narrow.
