So many good points on this thread, its exciting to see most certainly.
I actually contacted Lego a few years back about reviving Aquazone and the response I got was a little sad, but since that time they've started to hint more at old themes than they ever had. The blacktron emblem in the collectible minifigures and Space Police was a good example above, and the upcoming movie is going to revive a lot of classic themes as well.
That being said, I think if they do revisit any classic themes it'll be much like the upgrades through the various generations of Blacktron and Space Police. It'll be a new aesthetic following some basic elements, bringing the theme to a new generation.
And now to my votes... I had to vote for a couple, no doubt.
My top pick is Unitron. I felt Unitron were the spiritual successors of M-Tron. Both seemed to be an engineering corps, making it possible for the other themes to go about their business. Sadly, Unitron was such a limited theme with so few sets. Sure, they had one of the largest monorail sets, but they didn't get their chance to shine. Their uniforms are so darn iconic!
That being said, I wouldn't ever be surprised if M-Tron or Exploriens were revived before them. M-Tron had such a great catalog presence and I have little doubt that they could be modernized to great success. The red and black color scheme hasn't been used a lot recently, especially with transparent neon green elements. I know they could do a wide variety of sets and really go wild.
I'd like to see Exploriens done again, but they had one of the widest varieties of sets already and they might not want to rehash it all.
And now the bad guys... From what I've read on this board, Blacktron has more support than I ever knew and it has more of a foothold for a revival than any of the other sets (as we've established with the minifigures), but I reall want to see Spyrius back. They are so insidious, so conniving. I really wonder how they would look with a less angular design, or if that would under-cut their design.