Final fantasy XIII's finale saga in the series, see the return of Lightning in her very own apocalypse adventure, and what better way to end it then to bare witness to a matured Dajh, son of Sazh Katzroy. That's right, the little guy from Final fantasy 13 would make a welcome return in Lightning Returns: Final Fantasy XIII.
Imagine a grown up, bad ass Dajh. Considering you'll only be able to control Lightning through out the game, that eliminates a chance to control Dajh, but that doesn't eliminate a cameo in the slightest, better yet why not create an extra DLC focusing on a future playable Dajh. Final Fantasy XII-2 lessened father and son role possibly due to Sazh's increasing popularity, but Lightning returns could make up for that.