There is no such thing as "soft" or "de facto" canon in Trek. Ask one of the novel authors* if their novel is in any manner "canon" whatsoever, and they will tell you that is absolutely not. If it was a produced, aired television series or movie, it is canon. If it is a licensed work, it is not canon. It's that simple.
(* Seriously, go ahead. Several authors post on the Trek Literature forum at TrekBBS. This "soft canon" nonsense comes up regularly there, and they all shoot it down.)
Canon or not, whether you read them or not, the novels are all we have left of Star Trek. There is no new TV series in sight. STO is circling the drain, and when it dies it will be the end of Star Trek video games. All that's left is the J.J. Abrams reboot movies. If the next movie is profitable there will probably only be one more in the current continuity. Then Paramount will just reboot it all over again, and Star Trek will become like James Bond and Batman. 40 years from now there will be 5 different actors who played Kirk since Shatner. People will debate on the internet which actor was the best Khan, like they do now for the best Joker. And the version of Star Trek that lasted from 1966 to 2009 will be completely forgotten about except for the novels.
Originally Posted by Mayjer_Adama
Having Sisko in a flashback episode like the ones with McCoy and Scotty would be pretty cool to. Maybe even a new Dax on DS9 handing out missions
Why does it have to be a "new" Dax? Why couldn't it be a middle age Ezri Dax? She would only be 35 years older than the last time we saw her, and we all know that people live longer in the 24th/25th centuries than they do in the 21st. As long as she didn't meet an unfortunate end commanding the Aventine (yeah, I know, not Canon, who cares) she could still be alive and in Starfleet by the year 2409. I'm kind of getting tired of people assuming that every TNG era character must be dead by the time STO takes place.
As for Sisko, I will say again, it doesn't make any kind of sense for him to return after almost 35 years. Either have him return a few months after he left, like what happened in the books, only he spent the last 20 or so years living a quite life on Bajor. Or not have him return at all.