Make yourself sing/play aggressive/fast/hard songs so you become comfortable with them. Many people will avoid them their entire lives, but you can learn to do them great, just as you do more emotive, slower songs. This ability will help all you songs become more personalized and artistic - because you could play this song twice as fast as you're playing it now, you're able to see more details and possibilities, and make them realities.
Listen to music outside of your favorite styles and instruments to develop new concepts and to feed your creativity.