The only real extension for the SRT considered was one to Malvern (or Sheppard Ave at the very least). It does appear though that the alignment changed since originally the extension would have used the old Canadian Northern ROW but eventually they abandoned that in favor of following the highland creek and Progress Avenue. Interesting to note in the drawings I'll attach below show that the station at Sheppard would have made it impossible to extend the line to Malvern so I don't know what the play would have been. I am planning on taking a trip to the Toronto Archives later in the year when it is warmer sine I want to remake my SRT video so if I dig up anything new or interesting I'll be sure to share it.
While I cannot corroborate the idea of an extension of the RT downtown, I have seen that at one time the City briefly considered extending the RT along Eglinton Avenue and making the Crosstown an extension of RT. I can't say for sure when those discussions happened but it would have had to have been in around 2006 since that was when the TTC was committed to upgrading the RT to the Mk.II trains and Transit City would have still been in its infancy (it was announced in 2007 right?). It seems at least for a breif period of time the working proposal (or at least the napkin drawing) was for the Crosstown to be an extension of the RT using the new Mk.II trains. Obviously this plan was converted to low floor LRV's alongside the RT and even the idea of unifying the EC and SRT was brought up again this time using LRV's instead of the RT's light metro trains. Obviously this never happened because the subway extension became the preferred solution. The reason the idea of merging the RT and EC into a single line using the RT trains never went anywhere is no doubt due to cost since a light metro version of the EC would have the be grade separated in its entirety either underground or elevated, while LRV's could run down the middle of the street. I will also say I have only seen 1 reference to this idea so I can't say with 100% certainty that this was ever a serious proposal. When I start working on my Crosstown video I'll be sure to keep an eye out for more references to this plan.
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(model of the proposed stop at Milner)