We know that the season finale of Smallville - Doomsday - will feature the first time that Clark and Doomsday really get to throw down, and that two major characters will die. Now, according to Entertainment Weekly's Mike Ausiello we also know that there will be a major special guest who appears in the episode - and that same guest will be a source of major problems in Smallville's ninth season.
Ausiello says about the guest "He shows up in the final scene, and he's going to cause big problems next season." He continued on to say "I can confirm that there is a twist, but it doesn't involve anyone coming back to life." This suggests that the special guest is not, in fact, Michael Rosenbaum, returning to menace Clark as Lex Luthor. The emphasis placed on the word "big" suggests that the guest might be another hulking monster, but since he says that "it doesn't involve anyone coming back to life", most likely that won't mean that Doomsday somehow survives.
Other possibilities exist, of course. There are still a multitude of villains that the producers could bring to the show, or other major baddies like Braniac may end up returning. The guest might not even be a villain - certain heroes, such as Green Arrow, Supergirl and the Martian Manhunter have all caused big problems for Clark.
We'll find out for certain when Doomsday airs on May 14. Smallville airs Thursday nights on the CW. The drama about the early days of Clark Kent stars Tom Welling, Erica Durance, Allison Mack, Aaron Ashmore, Justin Hartley, Cassidy Freeman and Sam Witwer.