We know that there hasn't been a significant update to the line in ages. We also know Cook has publicly stated the line is not dead and that there will be something for pros this year. That's where facts end and speculation begins. For me, the big question is "what are they waiting for?"
I don't think it is a CPU; they could easily ship something with faster intel CPUs, and I doubt (understatement) they would release a pro line with Apple-designed ARM CPUs (that would certainly explain the wait, though).
So, what is it they are waiting for? Faster RAM? Unlikely. A terabyte SSD? I doubt it. Some retina 3D monitor? Unlikely; that could only cause delays if it were built into the hardware and I would not expect pro stuff to have a built-in display. Some interconnect (optical thunderbolt? Something wireless?) that will allow them to come up with a new form factor of a pro box? That, I think, is the most likely. But hey, let them surprise us.
I don't think it is a CPU; they could easily ship something with faster intel CPUs, and I doubt (understatement) they would release a pro line with Apple-designed ARM CPUs (that would certainly explain the wait, though).
So, what is it they are waiting for? Faster RAM? Unlikely. A terabyte SSD? I doubt it. Some retina 3D monitor? Unlikely; that could only cause delays if it were built into the hardware and I would not expect pro stuff to have a built-in display. Some interconnect (optical thunderbolt? Something wireless?) that will allow them to come up with a new form factor of a pro box? That, I think, is the most likely. But hey, let them surprise us.