If they came in and expanded SJP to 60k then that roof would be scrapped, could be a good idea now that it's been tarnished.
When I was on the stadium tour they said SJP was at it's max capacity. As one end is over the metro, then there's conservation orders about Leazes Terrace. Don't know the truth about that though.
Modern engineering will allow the Gallowgate end to be extended, this was planned before Ashley bought the club. The East stand can't have much done to it at all because of listed buildings behind it. I would imagine a few more meters at the top could be put on but it would never be cost effective.