I think there's zero chance you-know-who and you-know-who leaving to find you-know-who, will just be left there; I suspect a spin-off series. And I bet it's been in the works for a while now.
Filoni said in an interview that he wasn't thinking about the next series while he was working on the current series; his priority was to wrap up Rebels before deciding what to do next.
https://io9.gizmodo.com/we-tackle-your-burning-questions-after-the-star-wars-re-1823524129
So many people were obsessed with everybody dying [on this show]. I was really bewildered by it because I’ve seen the original trilogy of Star Wars that people like so much and it’s not like a death count type of movie. It’s a rather positive outcome, which I enjoy. I’ve always felt the best stories end and other stories begin and there’s no better way than to take two of my favorite characters and have them ride off into the sunset like I’ve seen in cowboy movies and Indiana Jones. And you just wonder, “What do they get to do? “I always like that in stories. One thing ends another begins, the story continues and that’s a saga.
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From the final moments of the show, when Sabine goes off with Ahsoka to find Ezra, it seems pretty likely that story could serve as the beginning of whatever Dave Filoni is working on next. However, when asked about his next show after a screening of the finale Friday, Filoni was very noncommital.
“Obviously, it’s intriguing,” he said. “It would probably make a good story... So I don’t know, we’ll have to see. There are so many stories to tell and I’ve been busy finishing this story. To think of another one while I’m doing that, that would just be not right. I have this crazy thing. I have to know what the story is about before I tell it.”
To which the person who asked the question replied, “You know what it’s about. They’re looking for Ezra!”
Filoni laughed. “No, no, no. It’s a bit more than that. So we’ll see.”
So for him, it was more about just showing that the characters' lives went on, because he thinks that's a better ending than just killing everybody. I think the ending of Rebels is more a story seed that's being left there as a future possibility. Series writers do that a lot -- give themselves options, set up possibilities that may or may not get picked up on. Filoni probably has (or had) to have conversations with Kennedy and Hidalgo and the network heads and the toy companies and whoever to figure out what they all want to do as the next show, and how it fits into the movie strategy and responds to the latest marketing trends and whatever. So they probably want to leave themselves a range of different options, rather than fixing themselves pre-emptively on a single path.