When the iPad 3 and 4 started to offer amazing resolution compared to the earlier models, people ran out of storage space fairly fast. Comics, movies, and apps are starting to take a large amount of memory and consistent file maintenance is often warranted. Beta Developers that are testing the new version of iOS 6.1 found an entry in one of the files that alluded to a new 128 GB model iPad.
Anything beyond that one fact is pure speculation at this point. There are always rumors of new Apple devices in development, but many people would love a higher memory model. Personally, I use Comixology and Marvel comics every week to check out the new HD comics. Each comic book these days are around 300 MB, and a normal SD version is 60 MB. If you have a collection of over 60 comics, that starts to add up. In conjunction with some purchased HD television and movies, your iPad is near full. Frankly, if Apple releases a 128 GB iPad, I would buy it in a second.