I don't really think a twin seat JF17 is really such a great idea. The JF17 itself is too small to make a worthwhile dedicated striker, so that rules out one of the main reasons for needing the twin seater.
For training needs, JL9s or L15s would be just as, if not more useful, for the PAF and any other prospective air forces because of the lower procurement and operating costs.
If we are taking about some time in 2015 or later, it would probably make more sense for the PAF to get twin seat J10Bs for a more dedicated striker along with the single seat J10Bs they are planning/hoping to get around then.
In terms of the JF17 development path, I think changing the engine to a more powerful Chinese engine, adding IFR, IRST, upgrading the FBW to full digital all axis, increasing composites to reduce weight and/or make the airframe 9G+, adding AESA radar, adding the option for conformal fuel tanks, and maybe integrating F16/J10 style mini-hardpoints under the intakes for navigation and targeting pods.
All of that could easily take 3-10 years to implement, depending on how much funding can be allocated, and they will round off the JF17 very nicely and keep it relevant for decades to come.