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.