I really hope to see a return of the Generals Challenges in Generals 2. They were a really great addition to the singleplayer experience.
That being said, I also hope Generals 2 will be released with subfaction Generals to boot. One way I hope for it be implemented is how Generals 1 was originally planned: players begin matches as their respective main factions and must choose between their three subfactions in-game before they are able to start building their forces.
It would really add to a dynamic play style that adds a bit of surprise to each players' experiences. Example: while anticipating say a tank assault from the other player, the opposing force actually chose the aircraft subfaction and begins the assault from the air, taking the player completely by surprise.