Will Fergus and Downey Die?

I rather doubt it-unless you count a couple of Gyro's clones, they have yet to kill of a hero character in the series. Of course, they'll undoubtedly be endangered by whatever's going on in the episode.

Since it is a kids show they won't kill off a hero.

It’s actually not unheard of: The Lion Guard had a hero character die; Star Wars Rebels had several. Other children’s series have had heroes die as well; and let’s not even get into Disney films.

What hero character died in Lion Guard?

Go check out the episode “Long Live the Queen”; I prefer not to post blatant spoilers from another series here.

I doubt that it will be a flashback episode, personally, I hate them. It's probably one of the F.O.W.L agents like Heron or Gandra Dee alongside Phantom Blot. F.O.W.L now tired of failing, must be going after Scrooge's family knowing that he would do nearly anything to save them (yes I know he doesn't have the best relationship, but after the episode with his parent's he forgave them and they forgave him). It would nice to see his Scrooge's brother and sister alongside his other family there. If it is a flashback story, it would be interesting to once again see his family's past, maybe even get into the roots of Huey, Louie, and Dewey's dad.

It could also be that one of the Missing Mysteries is hidden in the castle, perhaps even part of the Templar treasure that got alluded to last time.

That's actually a more realistic theory that I have not thought about yet, the Templar Treasure that they have not found yet could be one of the mysteries yet to be solved from Isabella's journal.

I want to see more Glomgold, Beagle Boys, and Magica.