Loads of room for it in this corner:
I believe filling in the corners, putting in the extra boxes, and extending the middle tier of the west stand cost circa £20m, so I imagine you'd be looking at a similar figure. Make the north safe standing while you're at it and hey presto 6,000 added to...
You really think police or a screen would be obstacles to expansion? The screen could go in the corner, or failing that they could just get rid of it. The police box could go at the south end.
I've heard all this said before but I'm sure there are ways and means, including relocating the offices in the North stand and taking it back another 20 rows. Obviously it wouldn't be cheap, but certainly possible.