Well, here's what I came up with.
Gambling: Find a way for Americans to gamble online somehow. How? No idea, but if it could be done, it would be incredibly profitable.
Porn: Biggest money maker online, but the competition must be massive.
Gaming: Not sure how much money is there, but I'm guessing it would take a long time to create a game and get a lot of people to join.
Sales: Need a unique item, or an incredible price point on something everyone else is selling. Possibly importing items and selling to resellers here in the states, but what item(s)?
Advertising: Good money, but lots of competition.
Web design: Lots of competition.
Something for the older generation: Old folks are quickly becoming the majority, what kind of web based business can take advantage of this?
That's what I've come up with so far, any other ideas?