Just a question to the experienced modders to see how difficult and/or possible this idea might be:

I was thinking about creating an extra terrain improvement not unlike the cottage, except when this one 'matured' (i.e. turned ino a hamlet) it generated a unit and then disappeared.

So...

1. Is it possible to do mod a thing?

2. For the idea to work in the way that I intend, these improvements would have to generate units that matched up with the civ that planted them, regardless of who's territory they were or were not in. I could see how this could be a problem....

Any ideas?

This could have really interesting ramifications for Sci-fi/Fantasy type mods. I can see Elves planting Treant Groves, Xenomorphs laying Egg Fields...good fun.
I don't know what could be done to implement it properly, but a place to start might be Eusebius' new mod (http://forums.civfanatics.com/showthread.php?t=187161), which generates units from improvements and removes those improvements.

Yes, this is totally possible in python and I'm sure, ofcouse, with the SDK. :)

Any ideas as to how...? ;)

Well, first you would use XML to add in two new improvements and set them up like the cottage works, so one turns into the other after X number of turns.

Then, with python, you would use the onImprovementBuilt event, check if the improvement just built is your upgraded form of the improvement, delete the improvement, and spawn the unit.

SOunds good! Thanx....