I was under the impression that once the North Stand is complete that although the cable net is still in place under tension that at both North and South ends it will no longer be supporting the weight of any roof structures. They will each have their own weight bearing beam arm structures.
I would speculate if that's true that if the West and East stands were made larger in line with size increases of North and South then the designers would have options to raise the height of whole cable net, although I suspect that would cause problems at both ends with insufficient height of supports as im not sure the newer roof support structure is sufficiently strong enough to add the cable net tension. (maybe not).
I thinks it's far likelier that each stand would have its own roof support structures. In any event there are design challenges no matter how its approached.
Personally I don't see any further size increases in my lifetime, that's hopefully at least the next 25 years.