From a purely accounting standpoint, there is no chance of a 5th gate. However, as Disney looks at the effectiveness (or lack thereof) of the Magic Bands and the new FastPass+ system, there may be a different response. A new gate may be the only way to absorb the huge crowds that are starting to be present in Orlando year round. If Disney keeps growing at 2-4% per year (as they seem to have been the past 10 years, including during the recession), they will begin to have a visitor base that they can't support in their current park configuration. If they average 250,000 people per day over the year, even at 2% growth per year, they will have 60,000 additional visitors in 10 years. That is not a number that can be supported in their current park configuration. Will the guest numbers continue to grow, who knows, but Disney would have to consider it a possibility.
I know it will never happen, but it would be interesting to see a breakdown of their actual visitor count, and what guests are actually spending in the parks (new vs. returners, and so on). Without that information, all of our thoughts are just conjecture, and we don't really know what makes sense for the company.
In the old days, a fifth gate would have literally been on the table just in case it was given the go ahead (EPCOT sat on a table for 10-15 years before it was actually given a go ahead). I would bet there are conversations and plans floating around just in case, because the visitor numbers imply that it may be needed in 10 or so years.