There will be more of a rivalry with TFC and the Impact than the NYRB and the Cosmos. Can't see people in NY caring that much, whereas up here it's basically the French vs the English. I've seen some mongs involved with TFC go down to Columbus and start fights and light flares in the stands. It'll be ten fold when Montreal enter the league.
Impact are already announced as an expansion team for 2012, Cosmos will (as I said, most likely) be the 20th expansion team.
edit: in hindsight it's clear you know this already, but still.