The area near the SR Technics hangar (I presume you mean the old EI hangars) wouldn't suit as the location for a pax terminal as there's very little room for outward expansion, due to the proximity to the new 28R runway. It might possibly work as the location for a freight terminal, however. I think that if the DAA was told to either use that area, or the 50 acre site it's just bought, west of 16/34, for freight terminals, it might make better use of the current parking areas for pax traffic.
The prospect of levelling the whole of the current pax terminal and starting with a green field site, just doesn't work. There needs to be a terminal and what is there now, as unsatisfactory as it may be in many ways, has to be worked with. However, in view of the fact that T2 won't be able to accommodate traffic requirements for much beyond its opening, the time to start planning the next phase of development is now.
The area between what will be the two main runways will be that site; the main terminal building, as I see it, will be close to the threshhold of 10L/R and the new facility will be designed with growth in mind, with satellites to be added as growth demands. With good planning (which means keeping the DAA as far away from it as possible), that terminal complex should be able to accommodate growth for the next 20+ years.