The roof structure is quite complicated and modifying it would be no easy task. Particularly the North/South stands as it is carried by the full span trusses (2 north, 3 south) which were assembled on the ground, craned into place, and then temporarily supported.
The East and West stands are supported by cantilever trusses and would, in theory, be easier to modify.
If there was going to be any expansion of the stadium capacity, then i think the East stand would be the place to do it, but it would not be cheap and it would compromise the whole aesthetic of the roof.
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