Im worried about something...

Not really much you can say about this.
Try designing and planning first, but if you are one of the people who decide not too, then build one side of the map first, and once that side is done, build a completely different thing on the other side.

 

No, do not do that.

Try to plan better and think out of the box, it will come to you eventually.

 

Maybe think of two symmetrical maps , cut them in half, and combine them.

But in all seriousness, I believe that planning is your best bet. Think of structures or ideas that you enjoy and jot them down. Once you beleive you have enough, try combining them into one map. Make sure balance is appropriately accounted for; don't make any single spot too "campable". Promote flow through the use of weaponry and map design.

 

The easiest way I've found to build asymmetrical maps is to start with an asymmetrical environment. If you start out with one side on paradiso and the other side in the cave, there's no way it's going to turn out symmetrical. It also helps if you build from one side to the other instead of jumping around the map. Start at one team's base and just build towards the other base and it should turn out asymmetrical.

 

Ha! I came across this problem recently. What I've discovered is that most asymmetric maps are built around a central point. Crown of Flies is basically two bases around the rockets room, same with Condemned.
I like to start with a interesting center piece and move outwards and around it.

 

When it comes to level designing, making an asymmetric map can get a little weird if you are not used to it. Because you have made many maps that are symmetrical, its natural for you to design a new map with the same characteristics. even more so, if you don't do any sort of pre-production planning.

I'm not sure what your work flow is, but drawing on a grid paper, or even using Google Sketchup can help tremendously on how your map is layed out even before you touch the forge.

@MyWhiteFlag: All maps, Symmetrical and Asymmetrical, typically have a "Focus" point or where the designer picked a mall piece and built around it. In my opinion, the biggest diffrence with Symmetrical and Asymmetrical maps are the pathings.

pyro6666 is also right. Part of the pre-produrction (at least in forge) is searching for the environment you are going to use. Will it be big, or small. Different things can lead to a more symmetrical map than the opposite.

Lastly, I know you want to make an asymmetrical map, but do not get discouraged if you keep producing symmetrical maps. Instead, just take a second to step away from the project, look at it and think "Now if I want to do this, how can I use what I have to my favor." or even think "I wanted that, but made this. When in the production did I start going away from what I wanted."

Its hard, but it is a mind game a lot of times

 

True, but I meant that while in Symmetric maps you build from base to base, in Asymmetric maps its easier to start in the center.

 

Something I tend to do is look at Bungie's maps and draw inspiration from them. Or sometimes even featured maps from here on FH.

Something that I noticed about asymmetrical maps is that they don't really play games with two bases, so don't try to design a map with multiflag in mind. Work on slayer, but don't give much attention to the red team's 'base' or blue team's 'base.' Just make them starting areas, and then they are forgotten for the rest of the game,

For one sided objectives, just make an interesting scenario but keep it challenging and fun.

EDIT: I'm a Senior Member now...