It's not hard to imagine. The cable giant is a force, and if its merger with Time Warner goes through may necessitate more hometown office space. It also has the disposable cash to pull it off. As a major American powerhouse, Comcast could have easily set up shop in Manhattan, but they didn't chose Philadelphia solely for our pretzels. Land is affordable and talent is cheap, at least by comparison.
A Comcast spokesperson told BizJournals it has no current plans to build a third tower, but similar comments were made during Comcast Center's construction.
On a related note, check out Comcast's slideshow of the new CITC. Notice anything unusual in the first rendering? The Mellon Building and Liberty Place aren't hiding behind Comcast, they've been edited out. Oh, Comcast.