The game will have support for creating and uploading ghosts, which are AI profiles based on the way you play Tekken. This has been a pretty cool feature in the previous Tekken games, and was one of the reasons why Tekken 5 was so good on the PSP. While I never really mistook those profiles for actual humans, they fight nastier than the default arcade-mode AI, so I'm guessing it'll allow for the same sort of challenge in Tekken 6. You'll be able to download and fight against any ghost you like.
The game has its campaign mode, which is sort of a brawler set in the Tekken universe, much like the Tekken Force modes of yore. Though the game won't ship with this feature, Namco is working on an online co-op patch for this mode. The co-op add-on will be free. Of course, it'll also have an online mode with rankings and leaderboards and all that stuff, too.
Tekken 6 has sort of flown under the radar for me lately since the arcade version felt too similar to Tekken 5, but I'm definitely warming up to the idea as its release date nears. Let's hope the online works well enough to satisfy.