The main reason I think that there's going to be more classes (or other kinds of packs, but what else can you sell?) is because they're adding an in-game shop. You don't do this if you're going to sell 1 DLC there.
I would love to see something like an Amazon. I really feel like spears and polearms are underused and underrepresented in the game. Although, I'm not sure if it can be literally an Amazon, since every hero has both male and female variations. But they can think of a different name/lore for a Dex-based class, while maintaining the focus on spears/polearms. I'm not sure about bows though as this niche is already taken by DH.
For Str-based class, I would really love to see a ranged-oriented playstyle. Not really sure how it can be done, but so far both Str classes are melee, so it would be a nice change. Maybe something focused on throwing weapons, which didn't really become popular with Barb.