The concept of art is absent from the game. What I’m thinking is making music and literature two mutually-exclusive tech trees. The techs of each would also mutually exclude themselves. I can’t think of appropiate bonuses for each right now; I’ll just throw the idea on to the dance floor and see if it dances with Maddona.
Music: the prime form of music in the Medieval times was the Gregorian chant. It was the forerunner of Classical music. It was the elitist kind, sung in Latin, in churches. Then, in the Rennaisance, it evolved into polyfony, and two separate styles came up: the motet, which retained it’s religious themes, and the madrigal, which was secular. So, in Imperial Age, you could have both as the mutually-exclusive. As for making a tech that would exclude Gregorian chant, it would be popular dance music. In that genre, there are quite a few: gigue, allemande, courante, sarabande, minuet, That’s only counting the European ones, so the mutually-exclusive could be dances from other regions.
Literature: techs should receive the name of literary works. There are a couple of renowed literary works from this period. La Chanson de Roland, the poem of El Mío Cid (as in the El Cid campaign), the legend of King Arthut (if I’m wrong here please correct me), and Don Quixote. Although Don Quixote was written well after the Rennaisance ended, there’s a connection. The Battle of Lepanto is featured in this game, and Miguel de Cervantes, the author of Don Quixote, fought in Lepanto. Also, there’s the Chinese work “Travel to the West”, which also comes a bit after the time period . So, once again, in Imperial Age, you can have those mutually-exclusive techs: “Don Quixote” and “Travel to the West”
You might be wondering, why make these tech tres mutually exclusive? This tastes like something that doesn’t belong in Age of Empires. The answer is: it’s art. The objective of art is creation or expression, and this is better reflected in making the choice of which tech tree you want to research.