The whole thing about names is complicated. The whole "Test Drive: NeXT" and stuff is derived from all the "Next-Gen" stuff. So if Test Drive took a freebie title involving "Next", it would take that quick and simple option away from the other "Next-Gen" games. Also, TDU2 wouldn't work because new fans wouldn't understand it - we all would, because we quickly say "Test Drive Unlimited" or "TDU" rather than thinking about what we're saying, and how "Test Drive Unlimited 2" wouldn't imply that TDU wasn't unlimited enough.
Also, I'm with Diablo on the whole hiding-the-names thing. A few years ago on one of the Atari official forums, someone was comparing NFS and TDU, and one of the members made a kind response explaining the differences, referencing how "TDU isn't NFS: The Island," and another made-up name. It was assumed that everyone knew he'd just made it up.
However, browsing the NFS forums a few weeks later, I came across a thread about someone asking about the next NFS game; this, of course, happens with any series (as we know :) ). However, the title he gave was "NFS: The Island". I have no idea whether he was a member at the Atari forum, or found it in a search, but it goes to show how knowledge, even though it may seem to be only what it is (in this case, a made-up example), could be taken the wrong way.
That was back before NFS: Carbon, so as we know, there hasn't been and almost certainly won't be any NFS: The Island. :cheeky:
Just my ideas about names. :)
-Leadfoot