PG talks a lot about this kind of thing in his essays (and book). (Zemaj's answer is pretty good, too.)

There are two models I can imagine for Twitter: 1) working in advertising, or 2) become a feature of some larger business.

You might also speculate that there would be some permutation of Twitter that would be interesting enough to become Twitter Pro or somesuch. But those kind of things are not often very exciting.