Couple questions concerning map generation and creating new 'actions'

I am not sure how to go about this as most changes I have done are in XML and are pretty minor. Like unit stats or something. But I now want to add in a couple things that I couldn't find info on in the forums. Or what I did find seemed vague and indirect because it actually dealt with something else that was merely related. So, I was wondering:

• I want to add an ability to all units capable to pillage to have a "pillage road" option. This will of course bypass any improvement on the tile and only pillage the road/rail on the tile. What files would I need to alter (outside of buttons) to add in this ability?
  
• Currently, whenever a city is founded on a flood plains, that flood plain is automatically turned into a desert. (So if the city is razed, you have a desert next to a river.) Is there any way to change this? As my current understanding goes, a floodplains works like a "forest". (i.e. You found a city on the forest, it clears the forest. Is there any way to make founding a city on a flood plains not clear the floodplains? If so, can you point me in the right area, please?
  
• AHa. I think I remembered what I was going to add here. I want to make a new modifier in diplomacy where if the AI comes up to you, and asks if you will join in on their war and you refused, it would give you the standard -1 with the asking civ, and a +1 modifier with the civ that you said you would not declare war on.
  
  Side Question
• When the game generates a map, my understanding it it goes through and generates the world, then goes back through sees where rivers are alongside deserts and grants them flood plains attribute. Then it picks starting locations. Then, for a reason I don't know, it will sometime generate a new river by you to make your starting position better. However, it does not go through and see if the new river ran along side any deserts. Thus you get deserts by a river upon turn 1. Is there any way to make it go back through look at the world again real fast and then have it apply the floodplains attribute to the new rivers it just laid down.

This is a last question is a side question because it seems like this would really get into some stuff where I am biting off more than I can chew.

Thanks.