Buying the cow
NCsoft didn't buy City of Heroes from Cryptic, they bought the complete intellectual property. Meaning that they invested in something more than just City of Heroes and City of Villains as we know them now. And while I'm sure they'll continue to support the original games with both free content updates and microtransactions, I don't think we'll see a full-on expansion for the game. Why not? Because a lot of the features coming in the free updates are expansion-worthy themselves. I'm certainly not alone in this feeling.
The long road
So when any company invests in something, they want to make sure their investment has both long-term and short-term livelihood. Part of the reason we see so many sequels in the videogame industry is because companies like to keep their intellectual properties as known quantities. Example: Sonic games may not have the best reputation as being wonderful experiences, but everyone sure knows about them and when new ones are coming out. City of Heroes 2 isn't happening just because NCsoft is feeling lucky or some such nonsense. It's on the way because they made an investment in a strong property and they want to see it grow into something more -- specifically, something that makes more money.
Trimming the "fat"
Yes, I'm aware that NCsoft is experiencing some financial rocky times right now. They've done crazy things like cancel Tabula Rasa while it was up-trending, which made no sense ... Unless they did it for a reason we're not fully aware of yet. I can't help but get the feeling that while one game was scuttled for short-term gain, other projects were receive more funding. Why would City of Heroes need more funding? It's doing quite good on its own.
When a company fast-tracks something, it requires some more money. And when you want to grab the attention of a crowd with growing interest in something like Champions Online, it's never a bad idea to announce your big sequel around the same time competition is launching. Raise awareness of your brand, draw attention away from the "next big thing" and remind people that you were the first and will continue to be the "best" superhero experience.
A new crowd
One argument I keep hearing is that people who're playing City of Heroes right now wouldn't want to move over to a second game. Well that's fine, I don't think NCsoft largely expects them to do so. What I expect is that City of Heroes 2 will be much different mechanically than the first game. You'll still have deep customization; something along the line of power "sets" and probably villain-play as well. Aside from that, though, I wouldn't count on things like combat, crafting and PvP to be too similar.
The reason for the difference is simple: NCsoft is going for a different crowd. They're targeting the same newer group of people that games like DC Universe Online and Champions Online are aiming at -- they may even finally make that long-promised console MMO. Don't hold me to that last bit, though.
You gotta spend money to make money, but spending it wisely is essential. Is a sequel wise? I don't know, it largely depends on how good the game is I suppose. I can say that a sequel seems all the more likely coming from a publisher who's produced Lineage 2, Guild Wars 2 and- oh hey, don't forget about Lineage III!
NCsoft is not afraid of making sequels to games they perceive to be prominent in the mind's eye of the gaming community. So don't be too surprised when they announce City of Heroes 2, and while there's always the chance that I'm wrong -- there's a very good chance that I'm right.