It didn't take me long to populate the world.
This is how I did it.
1. I have a lot of households and Sims I've played before and wanted to add them to this game to continue developing them. Adding them means my world is populated with Sims that already have skills and jobs etc. You just need to be careful not to add duplicate sims. If I find I've got the same sim twice in two different households (or the same household) I either use NRAAS mod to totally annihilate one copy, or I change its name and make them relatives thereby adding more interest to the game for me.
2. I have an ongoing family that I want to keep playing. There are about 130 members of that family and their friends I wanted to move to this new game. I used NRAAS Porter mod to pack then up and unpacked them in Twin Towns. Doing it this way maintained all of their relationships.
3. Occasionally I just want to make a new Sim and I do that and add them to the game. These new sims tend to have a combination of traits that I may have never considered using before.
In my current Twin Towns game I have a few Sims that were made early in Sims 3 life - when only the Base Game had been released. And there are others that I've made since all Expansions were available.
A reason why this gives me a diverse population is these are all Sims I've played before - even if I forgot about it. And I made them with different ideas and in different moods and for different games and that means they are all completely different.