cacruden...
Can you outline why you would do that? I just don't see the point when it would be more expensive to do your plan, and still leave the northwest of the city with horrible transit. One of the main benefits of the Weston line is that it finally gives transit to Rexdale, which is actually pretty dense and nowhere near any rapid transit line. Also, stopping at Cherry St. seems pretty strange. If you are going to take it past Union, you may as well connect it back to Bloor to form a DRL rather than a stub line that only goes a few stops.
As for the "Half the trains would turn down the weston line..." For one, the TTC isn't very fond of interlining. Secondly, that would mean that service on Eglinton West in the most busy part would be half that of what service would be in the suburban expressway corridor. There's no reason the area around Islington and Eglinton should have more frequent trains than around Eglinton and Dufferin.
Taking Sheppard from Downsview to STC I agree with, although I don't think it should loop back to bloor, as a line of that length is bound to cause more problems and delays than two lines would cause, and I don't think many would go east just to go back west, so transfers at STC wouldn't be as heavy as one might think. It'd be better just to have Bloor extended out there to meet up with Sheppard, but not merge into one line.
I just question the economic sense of building the Eglinton line now when it's almost certainly more expensive than the Weston one, quite possibly up to twice the cost, and may actually have less of a benefit.