I root for emotional side effect rather than physical, actually. This ability represents Luffy exploiting his inner child, being unrestricted, innocent, and dreamlike. This can also apply to the side effect. What if every time Luffy feels depressed or sad, it will hinder him from achieving more power? What if after G5, he will be drained from positive emotions, while knly emptiness or sadness remains?
This way, after being Hanuman, Bajrang Bali, Heroic God of War and Destruction, he is back to be a child who is in extreme need of emotional support. Thats where his nakamas come to hug him or pat his back or even smile and cry with him, or even thank him sincerely etc. I think this can be a great touch to G5, since it will re-instill the heart-warming vibe in the ability and even in the manga overall.