Aussie_Lurker
Deity
Both likely to happen.
I think exploration at Medieval also means that one of Commerce or Patronage is going to move to the Renaissance (probably Commerce). It looks more sensible to have 2 trees open at Medieval and 2 at Renaissance then 3 at Medieval and 1 at Renaissance.
My guess is this-Ancient: Liberty, Tradition, Honour.
Classical: Piety, Aesthetics.
Medieval: Commrce, Exploration.
Renaissance: Patronage, Rationalism.
Industrial: Ideology.