It sounded like the idea for the pack that Grant was enthused about was something of which the rest of the team had to be convinced. In my thought, Seasons wouldn't be something of which the team would need to be convinced, since it has been a staple. Any reservations about Seasons would regard implementation, I would think. Farming, as much as it is near and dear to my heart, it, along with other traditional things...hmm, Grant doesn't seem like a Farming kinda guy, lol.
Using that logic, and taking into consideration the things Grant is extremely amused and excited about so far--I'd say Superheroes is a pretty good guess. Superheroes are also very popular right now in the culture. If it continues on the same bent that the Science Career took, it could be quite well done. That Career is the Best Of All, IMHO. The Freeze Ray, the Invention Constructor, Transforming , Cloning, the almost super powers that Scientists have--all of these are things I bet Grant laughs in his sleep about. :D Imagine the mischief one could get up to in the game with superpowers. :o
It also sounded like this had been a fairly recent development, still a bit iffy. I would guess that Ep 5 has been decided by now. I also think that the team is very pumped about the work they've done on this iteration, justifiably so, if the bugs can be squashed. I agree some of the design work has been the best so far. To have TS4 continue another couple years on that note seems quite possible. Just some musings. :)