I disagree. They didn't (at least in my opinion) close out Morgan's storyline. My guess is that he'll back - whether in human form we will have to wait and see - but the next time he's back then we can draw it to a close. I'm 99% sure he'll return.
Warning: MASSIVE spoilers from the graphic novel below. It will spoil the end of season 3 of the TV show and it spoils the graphic novel. DO NOT CLICK IF YOU HAVEN'T READ THE GRAPHIC NOVEL OR DON'T WANT TO BE SPOILED ON THE TV SHOW. In fact, I'm going to triple spoiler it in case you accidentally click it.. and another spoiler around the massive spoiler.
YOU HAVE NO EXCUSES, YOU HAVE BEEN WARNED.