Yoga (asana & pranayama), Tai Chi, Chi Kung, etc are totally in harmony with aikido practice. Any type of mindfulness practice will benefit your aikido. Those type of (softform) internal arts compiment each other well. The key is to pick one that suits you.
Right now my focus is on Tai Chi, meditation, basic conditioning. I roll it all up into my aikido umbrella. Tai Chi gives me a way of performing kata (moving meditation) which I miss from my karate days.