It looks like the Mini 4 Smart Cover will be compatible, and I have that, so for now I have bought a cheap Mini 4 transparent silicone back cover that is Smart Cover compatible. I am hoping its flexible enough to allow for the larger volume buttons (I don't care about the Mic). I do already have the iPad Mini 4 Apple Silicone Smart case (back) but thats not very flexible so i'm hoping the clear one will be a better fit.
I have been looking on Amazon and some third party case manufacturers are already labelling Mini 4 covers as Mini 5 compatible so new mini 5 owners will have to be careful for a while at least picking a case.