Spells not for everyone
Something I'd like to see very much is that there are spells that are unique to a particular nation. Not because it is pre-programmed, but because spells available for research are determined randomly and different nations end up with different sets of spells (naturally different each game). Trading spell knowledge or/and stealing spell knowledge (when conquering cities) for example would become more meaningful, and knowing what spells are available to a different nation can change my strategy (rather than acting on the assumption that mostly all my spells are also available to all others).