One of my ideas was introducing Truenames to the players and the game; I was flipping through the Tome of Magic (we still run 3.5; we're all broke uni students, and we can't afford 4e stuff, though we all started playing on 3.5 to begin with), and I thought it would be interesting to center a campaign around the core idea of knowledge as power, and it would be a great chance for the players to get into some deep roleplaying.
Another idea I had for a new setting involved getting away from the way dragons are usually used in D&D. I thought about making them more relatable by decreasing some of their power and changing the way they're used by removing alignment specifications, but also setting them further apart through things like making Draconic a secret language. I wanted to make them more a part of everyday life in the world, and make them more approachable for both players and characters in the world, while still keeping them as a significant component of the game. I got the idea, admittedly, from watching How To Train Your Dragon; I wanted to create a lighter atmosphere for the players (we have two groups; in one, we play Eberron, and in another we play a fairly dark custom campaign) where they could still have the same level of action.
I'm hoping for tips and advice on building the campaigns and worlds more than anything. I've already ran some of my ideas past my players and they seem enthusiastic, so now I've got to put it together. I'd love to hear from people who have more experience in this department than I do.
Creating worlds can be rather daunting, though lots of fun at the same time. I should know I'm in the middle of creating a custom campaign right now. :P Something that I've always used that helps me is to write down a list of a few broad topics that I want to keep as sort of 'cornerstones' of my world. These can be anything you want, but they should be key concepts that serve as central anchor so that you don't try to incorperate to much and wind up with at setting that doesn't make sense. What this does is allow you to have a base to touch back to in case you ever loose focus and need something to referr back to.
So like you said before you want one theme to be 'Knowledge as Power'. This is a great theme to list as it is narrow enough to only give a certain flavor of ideas, but broad enough have a nice selection to choose from and mix together. Things I'd consider is who can use this knowledge and power? Is it referring to mages solely? Given the idea you had about Truenames I'd be inclined to say yes, but I could be wrong here.
After you have your list you should have an easier time getting a clearer idea of what you want your world to be like. It may not come easily at once and you may have to change you list a few time, but this works for me when I use it.
Once I get that squared away I usually like to draw a map of the area that I'm going to be putting the campaign in and start getting an idea of what sort of people and places inhabit the world. This also gives me ideas of historical events that I might referrence later on.
Lastly if all else fails, look to your favorite stories for inspiration, be it video games, books, comics, movies or television. In the words of one of my favorite artists, 'Good artists barrow. Great artists steal'.