I think EA/2K are paying more for the NFL/MLB licenses than the respective leagues could make from offering them to multiple licensees so I'd guess in part that's why the situation exists. Same with the college basketball; I'm they raised their licensing fees because they knew there was a company out there willing to pay as much as they were asking.
As far as 2K expanding it's lineup, I think the following are viable (as long as it's VC developing) :
Links/Golf game: I didn't play the XBox version but I hear it was pretty good. Think there's a market for it since the only other golf game is Tiger Woods (that I know of) and from my understanding it hasn't stood out much so far several years into next gen. Because it hasn't, the window for a 2K golf game might be pretty wide.
All Pro Football: Since Madden/NCAA
still haven't distinguished themselves next gen, APF is still viable imo. I still don't think the hardcore-endorsed fully customizable game some envision would be the route to go since the hardcore on the whole killed 2K8 before it could even get out of the box. I think 2K would need to make a game that appealed more to the mainstream, which imo would mean concentrating on expanding the single player experience, and tweaking what's already in place for online/multiplayer. Keep the legends since they're easily identifiable, except do what 2K does best to make the game unique - improve gameplay/blow out signature styles/improve presentation. Virtually representing Dan Marino/Barry Sanders/Gayle Sayers/etc. has a lot more appeal than people creating teams of 7ft tall linemen and naming them after their neighbors. Definitely give more control/customization options, but don't think a vanilla "Make a Football game" game will make it. There would never be some one "standard" that everyone would adapt to, as some think. The base game would need to be all it can be.
I think the window for another APF is so-so, because at some point the law of averages will catch up and Tiburon will finally start getting more right than wrong with their football games. I'd love to see another APF but not sure that it'd be worth pursuing for 2K.
Boxing: I think people are starved for a good boxing game. I think Fight Night R4 will be good and they seem to have a great lineup of boxers, but I'd wonder if the game will have any depth. FNR3 was extremely shallow, but it was made when EA seemed a lot less worried about giving people a quality experience. I'd expect FNR4 to be much better/much deeper, or at least it should be. The window for a 2K boxing game might be pretty narrow.