Hashmalum
Explorer
I like the idea of Signature Spell and Spell Thematics working together, but allowing any of your theme spells to be spontaneously cast is way too good. Signature Spell, as written, allows spontaneous casting of one spell. Adding theme spells to that means you could spontaneously cast up to eleven (one for Signature spell, plus one per each of the ten spell levels) spells for the price of two feats, plus the other benefits of Spell Thematics. That easily equals or exceeds the power of the epic feat Spontaneous Domain Access, which allows clerics to spontaneously cast any of their spells from one domain (which only has nine spells), and you get to choose every one of your theme spells--effectively, your themed spells would constitute a customizeable domain, except without a domain power.
I would take things the other way round, and allow you to add Signature Spells to your theme list.
I would take things the other way round, and allow you to add Signature Spells to your theme list.