Scroll to the bottom for tl;dr version
A few things I want to say before I speculate:
1) This is my first post. I haven't even played SotM yet, but I have all the expansions and the Enhanced Edition sitting 10 feet away from me. I'm just waiting for my cousin/gaming partner who just had a baby to free up some time to play :)
2) I'm a lifelong comic book fan and my favorite hero (you could probably guess from my display name) is Iron Man. I have a soft spot for power armored heroes/villains of all varieties and I'm really looking forward to playing Bunker and (hopefully mastering) Absolute Zero.
3) While perusing the forums since I first heard about the game, I constantly find myself confused by the background information that other players seem to know that I have no idea where they are finding. That being said, I apologize if the speculation I'm about to unleash is information that is already known to be true or false and I apologize further for speculating on Expansion #4 before Shattered Timelines has even reached the highest Stretch Reward level.
So without further ado: With the heroes of Shattered Timelines (un)officially known and one of the villains officially known and another unofficially known, I'm feeling less and less like Bunker and AZ's nemeses are going to be in this expansion. Also, I'm not sure that a time and space themed expansion really SHOULD include nemeses for those two heroes.
The first expansion covered street level heroes, the second expansion covered magic, the third expansion covers Jack Kirbyesque cosmic-type stuff, so I think the fourth expansion will delve into another very common comic book theme: super-science and technology.
It's different enough from the time and space stuff in Shattered Timelines to merit its own theme and it would be the perfect place for the nemeses of the three tech-based heroes (Bunker, AZ, and Unity) all of whom are as-yet nemesis-free AFAIK. What does anyone think?
tl;dr I think the 3rd expansion might be a super-science and technology themed expansion and include the nemeses of Bunker, AZ, and Unity, the three tech-based heroes.