Jordan Maison at Cinelinx says he’s heard that the next season of Star Wars Rebels will be the show’s last., and that it will be succeeded by a new cartoon. This does make a certain amount of sense, and cartoons do tend to have short lifespans.
There have been similar rumors in the past, saying the next cartoon may be set in the sequel era. Personally I find that unlikely until (at least) after Episode VIII, but it’s certainly not impossible.
This is just me speculating, but there’s also the possibility of a new series that takes some of the Rebels characters forward in a sort of rebranding, thus a ‘new’ show. Maybe Rebels becomes Rebellion?
In any case, stay tuned. We already know (officially) that Rogue One is going to come into play, and that certainly could play a role in the series endgame. I also expect, if this turns out to be true, we’ll heard sooner rather than later.
For the record, here’s what Pablo has to say. (He also apparently flat-out called it BS at one point.)