The most likely option for a post-credits scene is to have a preview clip from Rogue One. While frankly not the most popular option necessarily, from a business standpoint, it makes a certain amount of sense. It's the next film up at the plate, and will come out the year following The Force Awakens. It's a huge gamble for Disney: a standalone Star Wars picture has never before been attempted. How it is done remains to be seen. With Rogue One set in the past, it won't be easy, and it runs the risk of leaving audiences confused. However, X-Men: Days of Future Past (albeit at Fox, not Disney) managed to pull it off with its preview of Apocalypse thousands of years in the past, so it's not impossible. That, however, was much less of a risk. A lot is riding on this first standalone Star Wars film, and its success or failure could determine how many more one-off movies from the universe are made. In any case, done right, having a Rogue One teaser at the end of The Force Awakens wouldn't suck. The key will be to make it something meaningful, and not just have a random trailer. What there needs to be is a clip that gets right to the core of what Rogue One is about. Either introduce the main character, or the primary antagonist.