The prospect of an extension to the Doug Ellis (and I'm sure it would be an extension given the change in plan with the North Stand, retaining atmosphere, etc) is an exciting one, albeit with numerous barriers to overcome.
If the club can overcome these barriers, I would expect the structure to be built across the existing road, with the upper tier extended back. If they're smart about it, the new structure will allow for the widening of the upper tier concourse which, from memory, was horribly cramped last time I was up there in the 90s. There may even be a second row of exits with another concourse on top of the existing (hopefully widened) one. I'd also expect at least a couple of rows of Hospitality seats and associated facilities.