It would be awesome if they did announce it at E3 2014. I think they really are back into annual releases, so Mario Party 10 could be announced and then it would be released for the holiday season of 2014 or Spring 2015 (including the month of March), so yes, it would be fairly likely for them to continue this kind of trend.
Mario Party 9 - March 2012
Mario Party: Island Tour - November 2013 (but outside of North America, it's early 2014)
Mario Party 10 - Late 2014 or early 2015.
But then, it might be a bit too soon for the other regions, so they might hold it off a bit, unless they decide to release the next installment in North America first, with the other regions later.
I would say a good chance of it happening.