Definitely can't wait for D23, and yes it's probably very likely we'll get more official info and tidbits there. In regards to the theorized betrayal, a rebellious teenager type of thing would definitely work, I think I too would prefer to see Violet rather than Dash take the role of the rebel, though if they decide to advance time 15 years (a la Toy Story 2/3) then Jack Jack would also be an option. However, a rebellious teenager is only one of many possibilities, that was just the first that came to my head; other possibilities I can think of now is a divorce between Mr and Mrs Incredible that perhaps causes the kids to take sides and makes the family vulnerable for exploitation (Incredibles 1 was about a midlife crisis, a very adult-centric topic, so a divorce would follow in the same direction, instead of heading in a more kid-centric direction with teenage rebellion); perhaps a romantic suitor for either Violet or Dash isn't who they appear to be, and this causes them to have to make tough decisions on who's really their family (this could either tie in with teenage rebellion or be a more later-in-life type of thing depending on how far time is advanced for the sequel); perhaps someone in the family is being extorted and must create a rift against their will for the protection of their family; perhaps the kids are going off to college now, and Mr & Mrs Incredible deal with an empty nest / separated family (again, a more adult-centric theme a la midlife crisis or divorce); perhaps Mr and Mrs Incredible become old and we get a conflict where their children try to put them in a retirement home or something, but the parents refuse to accept the limitations and challenges of their aging (the old veterans vs the young hot shot rookies, a la Cars 3 a bit?); perhaps one of the family members die and this greatly impacts the dynamic amongst the remaining members (Nemo, TGD and Up have all dealt with death, so it's a common Pixar theme). And that's all assuming there even is a split in the family, which there may not be. So for now we'll probably just have to wait patiently.
Indeed, D23 is more or less is where we're going to get news from (I wonder if Brad Bird is going to make a special guest appearance but introducing himself Edna Mode style seeing as it's his film, he should have the honors.)
And that's one of the reasons why I don't want another retirement-like plot element for the 2nd film (it was more or less dealt with in the 1st film via the Supers Banned movement after the lawsuits and Bob's Mid-Life Crisis dealt with that retirement plot element perfectly as he missed the action and felt that his Glory Days were long gone and he would never get them back, and Bird wants to go new directions rather than reusing old ideas from the past as Edna Mode puts it best, although it could be built up and brought up at times leading to the climax as the kids take over to defeat the main villain whoever the main baddie is), splitting the family up via divorce was also brought up in the 1st film on the island as one of Violet's greatest fears that she doesn't want to face when Helen left them behind to try to find and confront Bob about him lying behind her and her kids' backs (a near-divorce-like story is a possible option if Violet rebels against the family which could split her family apart forcing her to confront her fears over her horrible decisions she makes, and could very well be our possible plot as it fits in well with the arc we got from the 1st film and wouldn't involve doing a huge time gap, plus a teen girl rebelling is far more believable to see on the big screen than say Jack-Jack rebelling as a 6-7 year old or even pre-teens), the college element isn't very popular in the Pixar world (it worked perfectly for Toy Story 3's plot device because it was set up thanks to Toy Story 2's story as it dealt with Woody's fear of Andy growing up and abandoning him later in life, but kind of failed in Monsters University as a setting and they haven't used it since then, but could be used as part of the 3rd film as a coming-of-age type of story, even though teenage rebellion with Violet could be counted as a coming-of-age story as well seeing as she would be forced to mature very rapidly to progress herself in order to bring her family back from the brink of tearing themselves apart while dealing with the villain that causes all of it to happen), also the young up-coming heroes vs the old worn out heroes would be far too recent to use (seeing as Cars 3 is dealing with that story already, so that's easily ruled out, although it could be brought up here and there, but not as the main focal point with numerous new supers popping up but not in Metroville but in other cities like New York, Paris, Tokyo and even Moscow (Bird also did say he wants to explore their universe more in-depth this time around and it's been awhile since we had a globe trotting focused story, although better handled than Cars 2), a death in the family would be far too dark to pull off and would probably upset the fans depending on which character they kill off (seeing as Craig T. Nelson is in his 70s now and anything could happen to him from now til 2019 (as we saw what what happened to poor Elizabeth Pena 2 years ago which now puts Mirage's possible role in the story in complete limbo), Mr. Incredible would be my guess as a possible death (granted a death to a character's family member/mentor isn't really that common as we have the other 3 films that could possibly deal with it in their own ways (along with Coco, which is clearly going to focus on death as a theme with Miguel hunting for his lost dead family and ancestors which could rule it out for the Incredibles II), even though Bob's story arc is more or less finished aside from Bomb-Voyage, whom he still needs to catch which is one the many mysteries that should be dealt with 1st (and fans are waiting to find out where he's been hiding after 15 years), remember kids are the target audience as are teens (and new fans who are going to see this film would want to see the familiar faces again intact, that's the reason we didn't get an older Nemo in Finding Dory, and fans were literally screaming for Teenager Nemo to happen in Finding Dory but Disney/Pixar stuck with the designs that made those characters pop (plus fishies barely look any different aside from being bigger as adults compared to when they are cute little tiny babies).
I'll wait patiently as we all will (I just hope and pray that they don't ruin the designs of the characters, sans Dash and Jack-Jack who are more likely to get design changes anyways as males get older they look completely different compared to when they were younger (I just don't want to see Violet's current design get changed as she ages to adulthood seeing as she's my favorite character of the series, seeing her getting a design change that makes her look like her mom in her post-time skip look would be a bit out of character for her (plus Violet looks better with longer hair than shorter hair as the longer hairstyle just makes her feel complete, unless they pull a Riley Andersen for her hairstyle if they want to slightly shorten her hairstyle without completely changing it.)
Also aging the characters with a big time skip would more or less kill Edna Mode off in the process and she's a very popular character in general (seeing as she was 60 years old in the post-15 years timeline, another 15 year gap would make her 75 years old and possibly be placed on life-support at this point, so someone needs to fill in on making super-suits and it wouldn't have the same comedic factor as it would with Edna).