StarCraft 2 has been a massive hit, and everyone knows it. The game came close to two million copies in its first two days of release, and there's no telling when it'll stop. That being said, there were a lot of things about it that weren't exactly new. The teams were the same, units as a whole remained intact (or were a little rebalanced). For as great as StarCraft 2 is, there's really not much 'new' content as a whole in there; it's really just an updated version of Brood War. This isn't a bad thing though, as it just means that Heart of the Swarm and Legacy of the Void will have a lot more features than we may expect.
The first thing that comes to mind is units. Re-balances will come over time with patches and expansions, but looking at the way single player campaign for Wings of Liberty is designed, we're more likely to see unit swaps in the next expansion than anything else. Plenty of units were removed from the multiplayer campaign that fans may want to see - such as Firebats. No doubt Blizzard will want to introduce more units that may not see the light of day for Zerg and Protoss as well. Rather than playing with a game engine that's been clearly well refined at this point, it makes more sense to fine-tune than change. New and returning units would change the game considerably, with little pressure being applied to the core gameplay.
Battle.net could also be receiving some improvements over time, leading up to the next expansion in or around 2012. Online video sharing, map additions, and custom game types are just a few of the options that will no doubt surface over the next few years. It's an impressive communication tool to be sure, but one that could use a little more refinement as well. Some social options just aren't intuitive enough, like searching a players rank, or viewing win/loss ratios on a team level, and naturally there are still a few bugs to work out (players disconnecting still being stuck in a party).
The biggest question for the next two expansions outside single player, is if Blizzard plans on adding any additional teams to the game. The series has spent over ten years holding three different factions at their core with no sign or indication of another race joining in the fight. However, things do change, and much like Blizzard has added new classes and races to their market-dominating MMO World of Warcraft, it's doubtful it'll be much longer before we see new factions appearing for StarCraft's world. There are simply too many perks to adding a new team into the fray. Much like adding new units to the game, a new team would bring a whole new dimension of gameplay to the field. Though the existing teams already have very well defined roles, there are still a lot of gameplay types that could be explored.
There's certainly a lot left for this franchise, particularly outside of single player. Letting fans know how far it'll be between both games certainly gives Blizzard a nice time frame to get things done in, and it's equally possible that said changes are already mostly done and ready to go. StarCraft 2 did take a massive amount of time to create after all, and the idea of making 'one big game' that gets split up into three smaller sections isn't that far out there. Still, RTS fans should have a lot of fun with StarCraft 2 for the next few years, and a lot more to look forward to in the future. We can all probably look forward to the next big StarCraft announcement at Blizzcon, where we'll be tortured with sneak peeks for the next few years. Assuming someone doesn't leak anything first.