While I don't really like the idea of "Hey, if you give us stirrups we'll give you crop rotation", a means of getting technology ... somehow, from other civs, does have its merits. As does giving a technology for the expected return on investment. Tangential idea, related to how distance works in a hex-grid opposed to a square one, a hex-grid allows for easy distance calculations since the distance between any two adjacent tiles is always 1 (square, with diagonals the distance is ~1.4142 units)... so, the idea would be for fruit resources or other perishable resources to have a maximum distance they can travel. Technology like curing or refrigeration would allow the distance that resource can travel, so giving a technology to a neighbor with resources just out of reach of its cities would allow it to get access to them and potentially trade them. Just a mildly tangential idea that's been eating away at me for a bit now.