The surprise reappearance!
Games that have been going for a while develop a stable of characters that your players have encountered (or played) in the past. It can be fun to bring one or more of them back to interact with the current adventure in some way. They can provide advice, or be an important part of the adventure, or be surprisingly up to something unexpected. It's a bit of a worn trope to make the old friend now a traitor, but it can be effective if used once. Potentially more intriguing is the old enemy who now has a reason to help the heroes.
Commentary by memnarch (who has not seen the movie)
The outlook now is definitely bleak, that's for sure.
Oho! So it's not BB going to do something, it's Luke! I wonder how he got his X-wing drained of water after all this time, but that's a small detail in the end. It's not like being in a swamp did that much damage to it back in Episode V. I guess I should have seen this coming since we had the Luke and Yoda scene. It would have been a little odd to not have Luke show up again.
So what's Luke going to be able to do to turn the tide here? I think I remember a Force Meditation thing that made the whole side fight better from the Expanded Universe, but that probably was discarded when the new stuff came in. Fly that X-wing better than anyone else and somehow take out the troops moving in? Lots of Force Barriers kinda like what Darth Vader did but to protect others? I think I can safely say though that it's not going to be Kylo's Force Time Stop thing.
Transcript
GM: Everyone got enough pizza?
C-3PO: Mmmf... almost.
GM: Recapping: The outlook for the Resistance is bleak. The First Order’s overwhelming firepower is on your breached doorstep.
GM: Suddenly...
Kaydel Ko Connix: I’m getting a radar contact. Approaching rapidly.
Larma D’Acy: What is it?
Kaydel Ko Connix: Something small. An... X-wing... Landing out the back.
Leia: An X-wing?
Luke: I walk into the room.
Leia: Luke!!
Luke: Leia. It’s been a long time.
Leia: You look good.
Luke: Yeah, I work out. Catching my own food on a tiny island. When you... How old am I?
GM: Uh... <flip flip>... 53.
Luke: When you reach 53 years old, you won’t look as good.
{beat, Leia looks at Luke meaningfully as he sits facing her}
Leia: We’re twins.
Luke: Just sayin’...