The big problem as always is Witton Lane - there a vague allusions to increasing the capacity on this side, but for anything to work they need to build out the back of the stand by at least 5-10 metres. It might be possible with the road realignment that will be part of the North Stand development, but they haven't given any indication yet of what might be possible.
Even if they could lose the garden area and move the road back, I suspect the next problem with be the light blocked to the houses behind.
Unless the club bought all of the houses I don't think they could make the stand any taller. They could make the concourses and hospitality areas bigger by building the stand outwards into where the road is currently, something similar was done to the NIA (or whatever it's called now) when it was revamped about 7 or 8 years ago. The images of the North Stand redevelopment certainly suggest Witton Lane is being downgraded into more of a shared space, so there may be some room available to do this in the next phase.