One way to make themes worth looking at for the true "system enthusiast" might be the use of prerequisites. In other words, just choosing feats are subject to prerequisites, but themes are not.
For example (a pretty wild hypothetical, so just focus on the intent, not the specifics) using my Sage theme:
Linguist (Prerequisite: Intelligence 15)
Master Scholar (Prerequisite: Trained in Arcana, History, Dungeoneering, Nature, Religion)
Ritualist (Prerequisite: Ritual Caster)
However, my fighter with 14 intelligence, no training in Religion, and no ritual casting can still choose the Sage theme (and theoretically is now able to cast Rituals to boot).
Just brainstorming. Any thoughts?
My original thought/hope for themes is that they let you bypass prerequisites.