Not quite MLS but Japan withdrew from the Copa America today and there are rumblings that however longshot WOW Canada might get the invite.
http://news.bbc.co.uk/sport2/hi/football/9446831.stm
Would be huge for the Canadian program and involve a round robin competitive game against Argentina - massive! Other rumours are that CONMEBOL is fishing for Spain to attend but that would not only need their approval but also UEFA's, or that Costa Rica or Honduras might get invited.
For the Canadian MLS teams this could mean they would lose de Guzman, Attakora and potentially Teibert and Dunfield for a long stretch this summer as the Gold Cup is already going on during the MLS season.
I think it is unlikely but am hoping for it - would arguably be the biggest stage Canada has been on since their last world cup.