I'm trying to learn more chords to become a better rhythm player. I can play barre and cowboy chords with ease, but simple major chords don't fit my sound. How can I learn more chords?
So you know major chords?
The next step are minor chords.
Once you have both major and minor, you will need to experiment with additions -
7ths, major 7ths, 4ths, 9ths, 12ths and so on.
You can use a tool like this
to experiment with different chords. A good thing to do, is to go through a tool like this, and try them out for yourself, see what the different chords sound like.
To be a GREAT rhythm guitar player, however, you WILL need to learn your scales. Once you figure out your scales, you will be able to figure out what chords will fit in a piece of music, just from the knowledge of what key you are in.