Kbrockman: I'd suggest that the first upgrade would be from Pratt & Whitney, giving more thrust or making the F-135 lighter or both. Engine technology hasn't stood still either since the design of the F-135 which takes it core from the older F-22 engine.
This sort of upgrade worked extremely well on the Harrier, and as further weight reduction on the Dave B will be difficult given the lengths already gone to, I see it as the priority.
With the advances in Avionics technology I seem to recall that the GR7 to GR9 upgrade actually lightened the aircraft whilst increasing the capability. Which was handy when the requirement to carry Sniper, a Terma pod and a useful weapon load was essential. Weight growth over the lifetime of an airframe is not a given, but more thrust is always good.