So there's been a rumour circulating for a while that Rebels may end after three or four seasons - meaning the end may be very near - and that the focus at Lucasfilm Animation will shift to a new show that takes place between RotJ and TFA. Even if this rumour turns out to be false, I think it's a safe bet we will see a post-RotJ animated show sometime in the future.
My feelings are a bit mixed on this. I've really enjoyed Rebels once it overcame it's rocky start and has become the silver lining to the dark cloud (for me at least) that has been the Disney acquisition so far. It seems like it just got started and now we're talking about it coming to an end.
On the other hand, the timeline between RotJ and TFA does intrigue me and I would like to learn more about it. I would also rather have Rebels have a pre-planned ending than suffer the fate of The Clone Wars - or a TV show that doesn't know when enough is enough.
Then there's the question of when it should be set and who it should focus on. There's a 30 year time gap between RotJ and TFA. That's the largest time between movies we've ever had. Should it be closer to RotJ and show the mopping up of the remaining forces of the Empire and the re-establishment of the Republic. Or should it take place closer to TFA and show Kylo's turn to the dark side, the rise and fall of Luke's Jedi academy, and the rise of the First Order? Or, heck, should it attempt to cover the entire 30 years with several time jumps?
As for who it should focus on, even though I enjoyed the shift away from main characters in Rebels, I think I would like to shift the focus back to Luke, Han, and Leia (and possibly Kylo) in the new show. There's a lot of unanswered questions about them and I think I would rather have that explored than be introduced to a bunch of new characters. And depending on who survives Rebels, we could see some of the crew of the Ghost back in the new show. I also think there's a strong likelihood that we'll get Thrawn back for the new show (assuming he survives.... ).
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
There is the utter east.