The Ontario Line extension to Sheppard will have a multiplicative effect on the usefulness of the Sheppard East Extension. With the Sheppard Line extended deep into Scarborough (beyond McCowan), and the Ontario Line extended to Sheppard, I imagine that Sheppard + OL would be a very heavily used route to get Downtown. In many cases, it'll be faster than travelling Downtown via Line 1 and Line 2.
And this is tangential, but I believe we should further leverage the Ontario Line by building BRT routes in Scarbourugh along Lawrence, Ellesmere and Finch to connect to the Ontario Line. Allowing busses along these heavily used routes to connect directly to the Ontario Line, while circumventing traffic congestion, will make the Scarborough transit network enormously more effective at facilitating Downtown-bound travel. Busses will always be the workhouse of the Scarborough transit network, and we're doing the region a disservice by not having a plan to make the surface network more effective.