I wonder if Din caught word about Tattooine, a battle that included both a jedi and a guy dressed in mando armor and sets out there? IF Fett does show up to help, he definitely would have info about the rebels and know people who had useful info. The empire may be dead, but scoundrels still wander the galaxy and still have lots of knowledge to share. Fett was a gun-for-hire before too, if anyone can help Din, its him.
Using Fett as a helpful cameo would really work and push the story forward, as long as it doesnt derail the task at hand with nostalgia. I'm thinking we wont get any Skywalker characters at all, Luke is out finding a quiet place to train Leia. As long as this new series isnt oversaturated with characters who already had a finished story, it will stick around for quite a while. If the guys from the Rebels series were left on cliffhangers, i'd be ok for them having it all wrap up here.