My problem with Agincourt isn't that it's not an obviously desirable connection, but that once you do that you're back to tunneling much of Sheppard, and salvaging the STC ROW becomes more problematic. Assuming an above grade extension of the OL is actually feasible I'm inclined to relocating GO down to William Kitchen Rd, where a deviation into a combined station might even be reasonable:
As far as taking the Ontario Line toward Finch goes, on the one hand it looks like we're well past mainline compatible equipment being on the table, so running to Richmond Hill Centre isn't a question of actually joining the GO corridor, while following Don Mills is the kind of elevated over road construction we seem to totally reject here..
If our actually acceptable options come down to the list below I see the most benefit in through running:
-tunneling Sheppard and abandoning the STC ROW while not linking the OL (possibly with intent to tunnel OL north one day)
-elevated OL following CP to Scarborough and through running with the Sheppard/RT corridor
-elevated OL joining RH GO north of Lawrence and terminating at Oriole/Leslie while Sheppard does some combination of the two
Frankly I think that one seat Malvern/Downtown is more of a marketing point than important to the network, but one that has a lot of benefit in terms of actually getting funding and support. Combine that with an emphasis on phase 2 being a Pickering extension that seems... as useful as such a project could be... and I think this is the kind of plan that could be a politically viable means to salvage the infrastructure and
actually get Sheppard extended.