I understand how you feel. I feel the same when I start a city, but once you have some growth, you don't really worry about the future anymore. You don't really need to plan much. I just think of a general idea of what I want my city to look like. I think of where I want my highways in the future, where industry and stuff goes and how I want the style of the city to be. I don't even plan where I want my downtown area, it just grows naturally from demand.
I don't put wide avenues for every street to "prepare" for high density (if I ever decide to put it, that is). I do put avenues, but not everywhere, just for areas I know I would have to do it in for the future. The challenge of the game is to work around problems that you've created from the past. Planning everything just takes the fun of that away. Over-planning also takes the flexibility of your city away. If you come up with a new idea of something to add to the city, and have already planned everything, it would make it hard to add it in.
This is especially good if you want an organic look and growth to your city.
So basically, don't plan. Build how you want when you want it.