Lots of good thoughts unfolding here. For my part, I think the three most plausible scenarios, broadly in order of my personal preference, would be:
1. Exhibition Place: a) there's room; b) it's central; c) there's transit (and will soon be much more transit), including, importantly, a GO station, which is critical given that a sizeable chunk of Jays fans (and other Rogers Centre event-goers) use to get to and from the stadium; and d) no one seems super jazzed about the current Ex Place masterplan.
2. Rebuild in place: The Jays have in the last few years, partly as a result of the pandemic, pumped tens of millions of dollars into both Sahlen Field in Buffalo, and their spring training complex in Dunedin, and have had to figure out the logistical concerns associated with playing in those places. I imagine there's a phasing program where you could figure out a way to demolish and reconstruct while only missing two seasons. Obviously, from the team's perspective, that would entail a substantial amount of lost revenue (and would probably also hurt them with respect to player recruitment), but at the end of the day this will not be the team's decision, and the revenue hit would be weighed against the accretions associated with redevelopment (especially if a full redevelopment included a broader vision for decking, resi, and potentially some other mixed-use). This obviously carries the biggest benefits in terms of retaining the current A++ location.
3. Portlands, specifically in the plot of land owned by the Federal Government (ownership map below, in the extreme SW corner of the district), which is to my mind the only parcel in the Portlands that might actually make sense when one considers size of parcel; number of owners (one); proximity to planned transit; adjacent future uses; city-building potential, and other factors. This isn't as desirable from a location perspective given that, even in the fullness of time, it would require transfer(s) from a GO station.
4. Downsview. I hate this option for reasons that others have stated, but I could certainly see the bean counters liking it. It's by far the simplest.