IF the Rumor comes true of needing to add either park hopper or water parks, try to check the park hours before making your decision which to choose, especially those traveling in September. Park hours are greatly reduced during this quieter time of year.
We have a 9 night trip booked in Sept and 7/9 of those nights the latest park closing time is 9:00 at Epcot with other parks closing earlier. If you tend to book 5:00 or later dinner reservations and think you'll switch parks after supper, it may not be an option. I opted out of adding the park hopper after reviewing the hours.
I'm sure it will still be a good option for many, just wanted to mention what I'd encountered.