They have mentioned a Mega-expansion on a few occasions, but haven't revealed how many decks will appear in them. Partially due to the fact that at the time they weren't sure how many they were going to place in there. Now of course since it is only two expansions away they may have decided at this point.
Is this the Rainbow Bright and the Color Kids timeline?
If the expansion after Time & Space is to be another base set, here's hoping it's Sentinels of the Multiverse: Golden Age. While the current heroes and villains are all seem to fit the Modern Age of comics, I'd love to see ten new heroes and four new villains fitting the pulp adventure and sci-fi archetypes. It could include the Legacy of the 1940s, Blade's grandfather, and other era-appropriate characters - robots, detectives, giant apes, jetpacks, etc. - all on sepiatoned cards.