The proposal to move the subway station under the existing SRT station sounds very expensive to me, and reusing the SRT structure also seems mostly unfeasible for subway. Also STC, is much more horizontally-oriented, as shown in the map Salsa posted.
That got me thinking, why not keep the SRT line east of Ellesmere, and build a new interchange and bus terminal at McCowan. The subway station could interface with McCowan SRT , and there's lots of land there to build a new bus terminal, perhaps integrated into a new development.
The SRT could then be converted to LRT after the subway opens, and extended east to Centennial College, and Malvern. Perhaps move Ellesmere to a new terminal just west at Kennedy, or extend up to Agincourt GO, or even to Don Mills Station as the Sheppard LRT. The existing Scarborough Centre Station could be kept, and the bus terminal below it redeveloped, maybe as an expansion to the mall.
Edit: I've made a map of this over in the fantasy maps thread: http://urbantoronto.ca/forum/threads/transit-fantasy-maps.3005/page-296