If in doubt, start with the archetypes on the pages. 70-73 of the Player's Guide (pages 104-107 of the first printed basic book) as a quick and easy guide. For example:
- "God / dess of knowledge:" Divination (Access to all knowledge) + Expression (Give or take knowledge) + Bastion (Protect knowledge)
- "Prophet or seer:" Diviniation (divine knowledge) + expression (words of truth) + relic (sacred object)
- "Magician / Warlock / Witch:" Adaptation (Ritual) + Divination (Scrying) + Subversion (Disguise)
On the basis of this list and descriptions of the theme book, I would suggest:
- If your mental agility is related to knowledge, perception, or deduction, divination is probably your best choice.
- If the mental speed allows a surprising creativity, the adaptation is well suited.
- Bastion or Mobility best represents strong defensive reactions, but be careful not to reduce your power too much.
In many ways, there is not a single true answer here – the theme books do not have a lot of "built-in" mechanics, so the most important thing here is to be able to write good power tags (interesting, multiform and helpful). . Read each relevant theme book and see if the questions asked match what you think are cool and interesting about the concept; if that is not the case, explore other theme books.