Mountains should not only improve defense and slow down army movement but instead be unpassable. In addition to the standard 3 types of land units, which i like to keep, there should be a 'guard' version of infantry and cavalry, maybe with double costs and upkeep and 1.5x combat values. No more sea transport of 50K troops in the early 16th century from Mecklenburg or Morea around the world to southern africa or malacca would be nice, too. Peace talks should include a map where you can click on the provinces you want to get or have to give away (i hate gaining a peace offer which sounds good only to notice right after accepting that there is one province left in enemy hands as an island inside my territory).