They found a way to shoe-horn a dozen shows in the past, surely they can do it in the future as well. My impression was always that Rogers Stadium was a test-run for an NFL stadium on that site to serve this purpose, as NFL stadiums are notoriously empty 95% of the year. Anyway, speaking of the NFL...
Ah yeah, the Rogers NFL team. Maybe it happens but we're talking about hypothetical things that seem increasingly unlikely. I almost would prefer a scenario where they continue expanding a multi-purpose BMO rather than building something new and duplicating the stadiums. But hey, maybe in the future there'll be a need for a 30K MLS stadium and a 60K NFL stadium in the same town (I think the NFL has a 50K permanent minimum requirement IIRC). Toronto's a big place afterall.
What would a "Big Event" be if not a concert or an NFL game?
As things currently stand there's no need for a larger multi-purpose stadium than what both BMO and SkyDome offer.