A few thoughts:
While I'm far from certain Toronto will get this, we do appear on most people and publication's lists.
The Unilever site would be an obvious choice given 'new build', large enough, transit connections (proposed)
While the portlands has the greenfield space, there is no notion of a direct connection to subway/RER which I think rules it out.
The Oxford proposal would close in on the right size and could be made larger conceivably, sitting virtually on top of Union Station it could also make sense.
Finally, if they were really open to the burbs, the only practical choice would be downtown Vaughan, and/or a massive build out on York's campus; but both seem unlikely to me.
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In our favour, easier access to a global workforce, lower corporate taxes, we don't double tax repatriated earnings, large enough, strong tech workforce, global airport, sufficient space.
Against us, optics in the US, and likely lower tax-incentive availability. I don't see QP or Ottawa opening the purse up on the scale some US cities might. Though I'm sure they'll throw something out there. Note I didn't say 'cost' as I think we're ballpark w/the other areas I see as most likely in the running.