Thinking constructively, I think we need new characters...
If we assume that the nine book series or any sort of mega-series like NJO is no longer an option, I think the next trilogy or duology or standalone has to star completely new characters. Look at Crosscurrent/Riptide, for instance.
Now what I am wondering is if Del Rey is going to put out a trilogy or something that's trumpeted as the next "big story," or if the post-FOTJ books we get are all on the level, so to speak, with Kemp's duology (trilogy?), i.e. no particular story is going to be given more gravity to it than any other, or Kemp's existing novels.
I think if they do have a trilogy or what have you that is considered "galactic threatening" or at least a "big story," it should definitely star new characters, maybe Ben and Jaina included, but not star Luke or Han or Leia.
Personally, I'm not a fan of Ben at all, and I don't have any desire to read a book headlined by him in the same way that I enjoyed books headlined by Anakin Solo (Conquest) and Jacen Solo (Traitor). The jury is still out on Jaina, as I didn't really get into Dark Journey back in 2002, and I haven't gotten to it yet.
I really, really, really think the books need to get away from being so focused upon the Jedi. Look at the last three stories set chronologically prior to the New Jedi Order series:
The Corellian trilogy
The Hand of Thrawn duology
Not a single one of them had a dark side threat in them, and Luke Skywalker and Mara Jade are the only Jedi depicted in any of them, and Mara isn't a Jedi in either the Corellian trilogy or Hand of Thrawn duology. I really think we need a story like this, and looking at the Corellian trilogy, the story isn't even "galactic threatening" insofar as that the threat is a much more personal one to the Solo family, especially the Solo children. Granted, there was a so-called "Starbuster" that rose the stakes. And Survivor's Quest is obviously not a galaxy threatening story either -- and I would argue that Hand of Thrawn is something in which preservation of the New Republic is at stake, but the Empire isn't guaranteed reconquest of the galaxy if the heroes fail, the galaxy would just be fragmented rather than mostly unified under the banner of the New Republic.
The books have definitely been lacking the Han Solo character, and this is in spite of Han Solo being in them, because he's been marginalized to a minor role due to the Jedi focus plus his age. If there is a Jedi character, having him (or her) be like Zayne Carrick would also be helpful, because it would not make the Jedi a "win button" in situations that other characters would struggle with, and eliminate conflicts resolved through Deus Ex Jedi.
And as a personal preference, I've lately found the Jedi characters to be fairly boring. The minor schism in the New Jedi Order series has been useful to make the Jedi characters interesting, and it's obvious based upon later inner conflicts among the Jedi (Corran vs Kyp, Saba vs Kenth) that the writers feel compelled to include these in order to make things interesting. Jacen and Anakin's conflict with each other allows them to be foils for one another and differentiate the two as male Solo Jedi, whereas Jaina was made a Rogue Squadron member to make her stand out. I don't think anything makes Ben stand out apart from his brief jaunt in the Galactic Alliance Guard, and that's not really a large part of his character.
I think these Jedi conflicts is treating the symptom rather than the disease, because I think the Jedi Order -- especially the one depicted in the prequels -- is boring. They're monks, and that really limits their diversity in character. Ancillary Jedi characters are all the same and you can swap their names out for one another. I think that's part of the reason why the Sith are viewed as more interesting and why we've been inundated by Sith novels; the Sith are all about individuality and any particular Sith Lord is very unique compared to any other, e.g. Darth Sion, Darth Nihilus, Darth Vader, Darth Maul.
I also think that the Galactic Alliance has become too antagonistic with Daala and with the focus on the Jedi, it's really ceased to be viewed as a force for good in the galaxy to the readers, but an obstacle for the protagonists. I suppose the Wraith Squadron novel will help in that regard to give a positive perspective to some of the people within the government.
And perhaps SW Legacy is handicapping the books in this regard with the way it has depicted the Jedi Order as being very much like the prequel order, but one way of mitigating the way that the Jedi lack individuality would be a return to the way that they were depicted in the NJO with Corran Horn being called up from his military reserve status, having Jedi serving in the Galactic Alliance military as X-Wing pilots or soldiers, but I suspect that's not much of an option any longer since the young generation of Jedi grew up being trained as Jedi and don't have other skills to draw on.
I think the biggest barrier to me, for the post-FOTJ novels, is characters that I'm invested in, and after 21 books with Ben, I don't think he's going to be someone I become invested in like Luke or Jacen. I think that's the trouble that comes with the way that the Jedi Order is structured so that the characters are trained from a young age, rather than Luke's early classes with characters like Corran Horn that had prior life experiences to being a Jedi that informed their characterization.
And I don't think it's necessarily unsolvable without inorganically going back to that -- though I suppose the post-SW Legacy timeline might allow for it. But the most interesting Jedi character in the New Jedi Order series in my re-reading thus far is Ganner Rhysode, and his depiction hasn't given the reader any inkling what life experiences he has had before becoming a Jedi, if any, and his character is depicted solely as a Jedi.
I think perhaps that would suggest that part of the problem with the books we've gotten lately, i.e. what Ganner Rhysode has that Ben and the other Jedi characters have been lacking, is a character arc. I haven't really seen Ben change all that much as a character from his experiences. Ganner changed enormously just over the course of two books, more than Ben has over the course of many times that number.
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