I've long thought that, If we could go back before the Sheppard subway was built, it would be better simply to use Finch as the northern crosstown route - further north, greater density (excluding the density which has developed between Yonge and Don Mills due to the subway), and Finch continues further west than Sheppard does anyways. This all being said, since there already exists a tunnel along Sheppard, I'm still more of a fan of turning the FWLRT southbound east of the Spadina line and then linking up with the Sheppard tunnel. The Sheppard subway will never be extended, and rightfully so, so rather than keeping it an orphaned stub we could use it as part of a fuller crosstown line.
Further north, outside of Toronto itself, the 407 Transitway will provide an even better northern crosstown solution, whenever that may get built. I've also often wondered if Steeles would be a good candidate for LRT.