Well before anything they just need to add PvP rewards back to the game. It's a gaping hole and huge problem and should be addressed before anything else.
Once that is rectified, I agree solo-Q is a great idea. I can see how soloQ arena would not work, in the sense that it could not really be the same ladder as regular arena. Arena is all about coordination and communication, at least at the bleeding edge, and a soloQ could not provide the same experience (or performance). It wouldn't be fair and cause chaos.
RBGs however, can absolutely and very easily become soloQ. Next to no one even does RBG anyway, so by default Blizzard has plenty of room to reimagine RBGs from the ground up. More so there is no serious RBG ladder; you will not impress many people with any RBG rating, other than those trying to pug.. RBGs. SoloQ is not just entirely possible here, it is clearly a positive direction to take RBGs in. It might even revive the entire thing.