Just because a ending isn't anywhere in MK endings doesn't mean it can't happen. We won't know anything about Sektor or Sereena until they appear as characters in a future MK game that explains what has happened to them the last few years. Most of the stuff in there GBA endings are speculation when it comes to Sareena and ect. I think we will see Sareena and Sektor in the near future MK game. I guessing highly on MK7 since there is much fan demand for it. In the end what is valid can only be proven threw a character storyline in the sequel or even a mention.

Hmmmm, well, I may be wrong (often am) but I thought the MKTE additional cahracters endings were canon, but like I say Im prob wrong.

As fro Sareena and Sektro, I think they will both be in MK7.

I reckon Sektor is biding his time in Japan amassing his new Tekunin army.

And I bet Sareena will appear now that we know Noob Saibot is Sub Zero and she is involved with the two brothers. She is probably one of the only people who have been involved with both, so could be integral.