There's no way they're going to go to the trouble of removing the roof, adding adding an additional tier above/behind an existing one(s), and then adding a new roof, with all the engineering complications, structural changes and expense that it'll entail, just for an extra 3 or 4 thousand seats. We'd be left with a tiny but very expensive new section which only contains about 7 rows, and still have the same problem that demand far exceeds supply. They've got to be looking at 40000+ surely.
Since we returned to the Premier League, every game at the stadium has been full (with the exception of part of the away end a few times), and that's with an expensive membership scheme that only allows people a chance at buying a single seat, and makes it difficult for non members to get a ticket at all. Selling 30,000 season tickets is a reasonable target. then another 10,000 for non-season ticket holders, away fans and corporate means that 40,000 minimum for most games is achievable. A fair comparison is Sunderland. Capacity 48,000, average Premier League attendances were around 42,000. Rarely went below 40,000, usually full for the big games.
With Swansea, Watford and Palace all looking to expand to 30,000 plus, and traditionally smaller clubs such as Brighton, Southampton and to a lesser extent Stoke, already at that 30,000 level (plus Derby and Cardiff City potentially coming up with bigger grounds than us), we need to elevate ourselves to the next level.
As recent Premier League Champions, the most populous city in the East Midlands, a huge catchment area with no major clubs to the South until London and none to the east until Norwich, and with all three local rivals currently in a lower division, now is the time to tap into that.