I think most of the Batman and Superman villains are golden age remnants ie shit came up on a whim. The ones who've survived the longest are generally representative of a character archetype or pathology that way they can be interesting and multifaceted without being caricature. It's one of the reasons why the regular Arkham rogues gallery is that good. They each are a pathology for which Batman is the cure and sometimes they play against each other.
Sidenote: The newest justice buster issue is very good
Mariko tamaki and Kelly DeCunny should never be let into writing capeshit ever again, never ever ever.
Superman's best villains are usually mirroring him thematically (Ultra-Humanite, Lex Luthor, Brainiac. All geniuses) or are foils to his virtue (Bizarro, Parasite, Zod, Metallo, any prominent Superman Rogues gallery powerhouse). The third category belongs to the Toymen and Pranksters who've just kinda been there forever. Mr. Mixy is probably the one notable sorta exception to the idea of being a thematic mirror or foil to virtue. Superman's rogues gallery is pretty concise in the really well known ones, but I think we still have room for more development for plenty of them.
Personally, I do think we've miled Luthor, Brainiac, and Zod quite a bit and could use a little break. I did like the Superman on Warworld run from a few years back. Mongul's always been one of my picks for big bad alien that Supes fights. Darkseid needs to be reserved for special events and none of the other big bad villain aliens carry that level of dangerous. Zod's matched with Kal, but has his own code. Kanjar-Ro is a Silver Age holdover. Sinestro's not part of this rogues gallery.
I guess that leaves. . .Despero. But he hasn't been used effectively since the JLA run right during Infinite Crisis. That JL Detroit story with him was bone-billing. The further JLI-JLE era tales with him were also pretty good.
I think we have some possible greatness left with Superman in the right hands. A run exploring Superman's presence and influence on the world around him in heartwarming ways would be nice. It's been done before, but I think a Superman run that explores the dawn of the "modern age" of heroes (I.E. Supes going public in metropolis) could be really fun. Have it be told through the lenses of retired heroes, villains, and adventurers of all kinds.
Another thing I also enjoy is that the idea of a Super-family isn't bad. It just needs to be handled decently
Letting Connor go back to Hawaii would be good. Let them all scatter around.
Stop focusing on weird gender/sex shit. Just give us heroes again. Wanna do a genderspecial bullshit story? Write it from the perspective of an outsider looking in. Have the lenses be the hero's. No shit people are gonna be weirded out with Jon and Tim and other characters just suddenly switching to being gay. 99% of the population isn't gay. The fucking '90s Northstar story about him coming out was far better executed because it told both sides, had an emotional core, and the reader was akin to being on the outside and looking in.
I hate how a lot of modern LGBT/idpol narratives put the reader in the mouthpiece's shoes because it doesn't work. The vast majority of the people on the planet can't identify with this, so don't alter and fuck up things to get your idpol wank on. Wanna tell me a story about how Captain America had a gay buddy from his youth in the '30s and '40s? Some dude in the 80s did a pretty good job with that, compared to what's out now. Wanna have an Ironheart that's likable? Don't make her into a fucking redditor. You can have your goofy idea work, but just do your homework and look at what made Kamala Khan likable. Like, fucking hell, it's not that hard. She's 100% a token character, but they handled her in a way that made her likable. Miles and Sam Alexander were also likable. It's not hard. Amadeus Cho, for all his bullshit and annoyances, is at least an understandable character that's been given time to grow into a deeply flawed hero. Ironheart's a jackass who didn't earn her spurs and spent her first impression being a little bitch.
Also wish we had big anthology books again that'd rotate featured stories. I want more Plastic Man and I've read literally everything with him.