I do not think a fifth gate is likely for awhile. The question is not theme, but type of park it needs to be to get built. Look at the current parks:
Magic Kigdom- Park dedicated to all things Disney
Epcot- Park dedicated to history of our world/other countries and the exploration of it and science.
Hollywood Studios-Park dedicated to all things movies on film and behind the scenes.
Animal Kingdom-A zoo done Disney style.
Each park is different, each with its own idendity. For them to add another park and bring in more guest than the other parks already are bringing in the park needs to be different than the others. The only type of park that Disney World really doesn't have is a thrill park.
The problem is, I think a thrill park is more money than it is worth to Disney and the reason they have not built one yet. Living by Cedar Point, I have gone there every year for almost thirty years and have seen and read alot about it. One thing with thrill parks is that there is alot of competition. With Disney, there is Disney and Universial. Not many theme parks are like them. With thrill parks, there are many, many more all over the United States. From my home in Detroit area, there are four or five major thrill parks with four hours drive. Granted, a Disney thrill park would be much more impressive, but that brings me to the second reason. Thriil park patrons really do not care about theming, they are there for the ride. It doesn't matter if the park has a villans theme or little baby kittens, if the roller coaster is 500 feet tall, they are going to be there to ride it. Cedar Point brings in millions of guest every year and the park has almost no themeing except in the kids areas. Third thrill parks are only as good as there last ride. Thrill park patrons will drive for hours to ride the next biggest and greatest roller coaster, but as soon as something bigger comes out, they are off to a new park. To keep attendance up, thrill parks have to bring in new rides every year. Last, the rides in a thrill park ar alot more expensive than most of the rides at the other parks. The cost to design and built a roller coaster is very high. Plus it cost alot more to run, you need more cast members to operate and check for safety belts and such. I know that Disney already has roller coasters and they are fun to ride, but none of them compare to most of the roller coasters at a thrill park. Plus to be competitive, they would need alot of big ticket rides right from the start. Cedar Point has 16 rollercoasters of different degrees, three large water rides, and alot of other thrill rides. To be competitive, they would have to build alot of rides. Most of the thrill parks have been around for many years. Cedar Point has been around for over 100 years.
If Disney were to build a thrill park, people would go to it. Especially alot of the teens that are on vacation with their familys. But I think that most of the guest would be people that were already going to be there. Disney is not going to build and staff a new park if it is only going to be thinning out the crowds at their other parks (unless we have finally gotten to the point where the other four parks are running at full compacitiy).
I think a villians addition or land at somewhere like Hollywood Studios with thrill rides would be more likely than a fifth gate at this point.