Jack needs to come back. He was definitely underused in the first season. I'd love to see him become a full companion again. Plus maybe we'll learn what happened to him during the two years of his life that were wiped. Seeing as Moffat was the one who wrote that episode, he probably has/had a plan for him, just like river song.
"The universe has to move forward. Pain and loss, they define us as much as happiness or love. Whether it's a world, or a relationship...everything has its time. And everything ends."-Elizabeth Sladen: RIP.