We have 30 years between the ROtJ and TFA which is leaves more of a chance of them doing something like Jumping the timeline of Rebels from before ANH to after ROtJ than cancelling it for the sole purpose of creating a new series.
I don't even think it'd be necessary for them to avoid the classic OT period covered by the movies. I mean...Lothal (and whatever sector it's in) seems pretty isolated from the rest of the galactic goings-on. It'd be reasonable to set a "flash forward" series during, say, the period between ESB and RotJ, and show the adventures of some sort of rebel team as they track down the DS2 plans or some such thing. I could easily see Sabine falling in with the Alliance Special Ops teams.
I honestly do have an expectation that they are considering this for Rebels in a later season.