Fantastic post overall. These are the ones that stuck out to me I'll comment on:
-Wonder Woman's family, like her rogues gallery, feel like a mishmash of surprisingly good and/or fun varied ideas that just need development instead of being abandoned after a single story arc. Donna should keep her origin as an orphan rescued by Diana and empowered by the Purple Ray/Amazon magic - a mortal origin lets her comment on Amazonian culture or Diana's viewpoints a la Robin being used as an exposition excuse for Batman, and I believe she has a "valley girl" (if I remember right?) or at least airhead personality that could be fun to contrast to (Golden Age, OG, etc.) Diana's serene and unflappable goddess persona. Make them close by Donna considering Diana her savior and Diana considering Donna her biggest success story as a heroine. Cassie and Artemis work just fine in their teams... though Cassie feels like she's meant to be THE Wonder Girl and nothing else the way Tim struggles being anything but THE Robin, but I do think Artemis being The Angry One actually works nicely in any Wonder family and a good running element would be them having to hold her back from wrecking shit. Agree on Nubia, and I always really liked the retcon of Hippolyta being the JSA's WW (especially if she keeps the Golden Age costume versus Diana's modern ones). It can slide in nicely to her blessing Diana as Wonder Woman the title, revived, as she goes into Modern Man's World.
Donna's got a more sociable personality than Diana, but she's also someone that has a backstory that can be edited for a reboot because it's kinda in flux.
Wondy's rogues gallery is funky. Circe/Hades are the big myth related ones. Dr. Cyber/Poison are the science-fiction ones. Dr. Psycho hasn't had much in the modern times aside from the Infinite Crisis era Secret Society. Then you have Cheetah, the one villain that's been labeled as a major Wondy villain over the years. Also, the Duke of Deception was a recurring foe through the late golden age through the silver age.
Nubia could be repaired and be part of the whole multi-ethnic Amazon enclave. Toss her into the background of the DCU as a Global Guardian. The Latina Yera Flor seems to be a similar case.
Maybe they should just say fuckit and give us an Asian Amazon to round out the ethnicities. Have them pop up as background characters or in a tryout backup feature. It seems trying to give one-off characters their own book almost never works saleswise. On the other hand, Nubia was mishandled when they tried to make her a thing in the past few years, resulting in a lot of backfiring. The fucking black firestorm and asian atom didn't recieve this much vitriol from fans because they were portrayed as characters first and weren't shoved into the ESG spotlight.
-Aquaman's primary supporting Aqua-hero seems to be Mera alongside Garth. Really, it seems most of them exist to be the Aqua member on every generation of Titans, not that I mind that, but I want them each having a niche in the Aqua 'verse itself. I struggle to figure that out for them save Mera as the wife/main partner and Garth the sidekick grown up. Hmmm... Garth is supposed to use Atlantean magic, right? Flip the "exposition" so Aquaman, who was raised on land, turns to Garth for knowledge of Atlantean history, magic, ancient evils (that they're conveniently fighting that very story), etc. Perhaps Lorena (Tim's gen) or Jackson (Damian's more or less) act as the human/reader viewpoint to Atlantis as well?
They've let Garth be the magic guy in the Aquafamily.
Aquaman's the center, Mera's always there with him, Garth's the magician and old partner. Then you have Tula as a dead hero. Lorena's from the 00s, but she's also specifically created for the whole Sub Diego story. Jackson's the '10s one that's always shown to be super new these days.
If we want to expand the Aquatic heroes shit, there's also Neptune Perkins and Tsunami from the '40s Young All-stars that have been around. Dolphin's Garth's Wife (who may be alive these days? IDK). The Sea Devils are the sorta normal human sea adventurer group. Lagoon Boy was a late YJ hanger-on or sth. Red Torpedo was a golden age hero who had an advanced submarine and has been referenced in Aquaman stories before.
And, if you want to really grab other Aquamen, we have the '00s Aquaman that showed up after Infinite Crisis, only to be confirmedly kinda fucking off because he believed he wasn't worthy. We also have, recently due to Geoff Johns' Stargirl: Lost Children, the 1940s Aquaman being confirmed to exist now.
The marine hero sphere has enough to work with in all this. They also have enough supervillains between Ocean Master, Black Manta, Fisherman, and all the possible Atlantean ones they can make.
Topo the Octopus would be a hell of a revival though.
-I actually agree 100% on those elements being part of a mainstream DCU, but using them as supporting heroes in an A-list hero/book's story arc as you mentioned is exactly what they should be used for, and their own focused stories if a writer gets an idea as part of the said anthology books, back-up strips, miniseries here-and-there, etc. And I am willing to bet you know this, but it's funny you mention Plastic Man since Elongated Man was created to fill his niche as a Flash backup strip character till they realized they owned Plastic Man anyways.
I have some sympathy to the companies themselves, surprisingly, in superhero bloat. Of course you need new ideas and characters to keep readers hooked, and you never know when you hit the next truly sensational character find of (X year). But DC never committing to a true reboot means it just keeps getting all messed up continuity-wise.
Keep both Plastic Man and Ralph Dibny. Ralph's become his own character. He's not a major powerhouse like Plas, but he's confirmed to be one of the top detectives in-universe and his marriage to Sue was always very heartwarming. He's a dude from the '60s silver age that's always running around with his wife like they're a sort of superhero version of Nick and Nora Charles (The Thin Man).
Plas is someone that's really fucking funny in continuity. They haven't retconned his origin to be at the start of the modern age like they do with everyone that began in the Silver Age (and the trinity+aquaman/green arrow). He's still a Golden Ager in-universe. He's still officially worked for the FBI, JSA, All-Star Squadron, etc. Woozy Winks is somehow consistently also as old in-universe.
It's also fun because Plas is basically kinda stupidly powerful and might be immortal (Morrison JLA era sorta went into this).
DC has proven that they know how to raise the profiles of C and B listers before. They spent years getting the JSA, Martian Manhunter, Green Arrow/Black Canary, and The Titans to be B lister books in the '00s and then proceeded to squander all of this with the n52. Then they basically tried to focus on getting the CW heroes a higher profile, which only really worked with The Flash, Green Arrow, and maybe Vixen.
I'm not sure what their priority has been in untangling the fucking idiotic need they seem to have for expanding the Bat/Super/Flash families. You have books, you have decently high profile characters that were selling pre n52 before you decided to "forget" the JSA/Outsiders/Titans sphere for almost no reason.
Pre n52, you dropped the idea of a Booster-led JLI right before fuckin' Flashpoint.
I don't know if it's Dan Didio's fault, ESG, or just idiocy, but how the fuck has DC let Marvel run off with seemingly everything in the '10s. The only reason Marvel didn't get very far is because of the retarded ESG-pushers and mishandlings of IPs. (The Fox- Ban, Spider-Man being shit since Civil War, 2010s pushing of every avengers related thing, trying to shill ESG heroes, the Inhumans push, death of Wolverine, Krakoa Era, MCu Phase 4 kinda bombing etc.)
I'm a lifelong JSA fag. One of the first comic book paperbacks I owned were the reprints of the JLA/JSA "crisis on earth 2" books. The whole legacy hero thing in DC is a good storytelling piece if handled right. I'd rather they not invent a dozen new Bat-family members and just focus on the cores of each legacy family as they were pre-n52.
Gimme a Hawkman and Hawkwoman that remember fighting the Amazons but never bring it up until we get another invasion and Wondy's like "you did what now?"
Man, the art from the DeMatteis Dr. Fate run is kinda fun.