To balance the traits, you have to think about the added value in terms of game play. A trait should not simply be an advantage. It also has to induce a particular strategy. On the other hand, it should be designed such as multiple strategies remain possible.
For example, your idea of giving +1

to expansive doesn't really add something to the game. Reducing the civic costs for Imperialistic also sounds like Organized. Of course, reusing the ideas of other traits make them simpler to mod (you only have to change the XML. Nothing to change in the python code...).
However, I saw some ideas that I really would like to try, if somebody could mod them :
1. +50% golden age length : a golden age economy would be possible ?
2. +1

on sea tiles
3. reduced unit supply costs
4. extended city radius by 1 tile (this one might be too strong however)
5. free or reduced unit upgrades (reminds me of Civ II Leonardo's Workshop)
6. +1 diplomacy bonus
Some of these ideas have the drawback of being so specific that they wouldn't be flexible enough. I mean... The game would be boring if you had to play the same way every time you have a certain trait. For example, I thought about a boost to cottage development speed, but that wouldn't leave any other strategy than cottage spamming.
Other ideas ?