It is a great idea to go out and shoot with a theme in mind because it forces you to think in a different way. Like an athlete that needs to exercise to stay in shape, a photographer also needs to exercise to keep up their skills. Often when I go out to shoot on the streets I know my first few shots are warm-up shots to get me ready.
I go out and photograph with a monthly Meetup group and there is always a theme. I use this time to do my exercises and in January the theme was shadows. I enjoyed this challenge more than I thought I would. I am going to add more themes into my shooting routines to get my mind thinking differently.