@rteetz comment about what Walt could have done with the proposed St Louis indoor park got me thinking more about this - and with a lot of time to think while driving over the holiday, probably thought in more detail than is needed, but hey
What if the 5th gate was an indoor park building off of the original idea Walt had for EPCOT. I know in the past when they looked into it building a dome that big was cost prohibitive, but was thinking that is a large hotel was part of it - that would include DVC as well and also perhaps some permanent residences (condos, etc.) to help offset the cost. I would think there would be a lot of people willing to pay a premium to stay in a hotel that is actually inside a park and a group of people that would pay a lot to actually own a residence inside of a park.
With this, it would be perfectly climate controlled allowing them to control everything. Part would then be to perfectly represent different seasons in different parts of the park so the permanent residences would have some seasons and decorations, etc.
- Villains Land .... with thrill rides and meet & greets and every year they would decorate that area around Halloween time
- Holiday Land ... featuring artificial snow (perhaps snow tube rides, etc) and then a version of a matterhorn like bobsled ride and obviously would go big time with Christmas Decorations
- Summertime Land ... have some traditionally summer time related attractions but main feature would be a water park which would be an idealized version of what River Country was. This would bring something unique to this park by having a waterpark element included with it and may appeal to people that would like to spend some waterpark time but don't want to take time away from the parks/pay extra to go to Blizzard Beach, etc.
- Yesterday Land ... would feature "forgotten characters" (such as Oswald, etc.) and also old attractions brought back but modernized and "perfected" - so Mr Toad, the Skyway, Horizons, etc.
- Pleasure Island ... would harken back to the original Pleasure Island at DTD but also have Pinocchio references (perhaps a ride, meet and greets, etc.) and include a new Adventurers club. This would be the main dining area for the park
Keeping with the original EPCOT vision there would be a big use of WEDway people movers - thinking parking structure would be outside the dome and you would ride in WEDways to get into the dome/the park.
Like I said, probably spent too much time thinking about this and it probably isn't very realistic but do think that if they do a 5th gate it needs to be something different and unique and "big"