I honestly wonder if they’ll do anything further with Luke (and Grogu) in this time period.
Grogu, I think yes. I don't think Mando's promise to Grogu that they will see each other again was just for his benefit, but for the audience's as well. Grogu is important to Din, and we'll get to see more of it. Luke, is harder because I don't think they want this to be more Skywalker saga. But at the same time, just as you might get married and start a family doesn't mean your parents and grandparents cease to exist. You see them from time to time, and they can still impact the direction your life takes you. Skywalkers, Solos, Thrawn, Emperor Clone are around somewhere. So I wouldn't be surprised if we see Luke again (although, I think if we do, that will precipitate a recast. This episode gave happy closure to Mark so it allows for the opportunity for him to leave Luke behind. Although, I could also see if they could find an agreeable actor, to still dub in Mark's voice)... but I also wouldn't be surprised if we didn't...on this show.
As for Grogu's future, I can see two paths. One is that like other MIA Jedi, he disappears into the Universe. Maybe what's best for him will be to find his species' home planet or a home planet for him and leave the humanoid politics and Jedi training behind. Maybe his blood is too important and for his safety he needs to be away. Some reason for him to move beyond all of this. Second, tragedy is in his future. I don't think it will be at Ben's hands, though, but involving whatever happens with Din. By telling us that Grogu has had many masters, I think was a tell that his Jedi education may not need to take that long, and so he will have left Luke's Academy by the time Ben destroys it. Ben Solo supposedly is born 5 years after RotJ, the show is 5 years after RotJas well. Unless, there is a huge time jump at some point Ben will still be a kid by the time this show concludes, and they won't keep Grogu away that long. I semi expect that we'll find out his species has a growth spurt depending on how much control of the Force they have. His growth was stunted, so he remained a baby. But Din's love and Luke's training will have unlocked his true potential. So he can reappear as an older, able to speak and participate in the fight version (that hopefully still sells merchandise well).
Also unknown (warning fan-fiction ahead
), is how much and in which show Filoni wants to try and fix sequel issues. Maybe, we see cute Ben ala Jake Lloyd Anakin. Maybe, he really bonds with Grogu and gets really upset that he's gone and blames Luke. Maybe Luke blames himself for whatever might happen to Grogu too. Maybe, that leaves them both fragile and distrustful, setting the stage for what happens next. But anything like this, I don't think that will be the point of The Mandalorian, but may play out elsewhere at another time.