Probably the biggest thing DS9 managed to salvage from TNG's atrocious original concept of them was the Ferengi, honestly, and I don't say that lightly because the Ferengi were fuckawful in their first run. they were trying SO hard to introduce 'the next big threat' among alien empires, to put them in the same conversation as the Romulans, but like... it just super didn't work for so many obvious reasons.
Which is why I find it fascinating that through all the characters affiliated with Quark's bar, the Ferengi were kind of rejiggered to demonstrate the alienness of the Federation from the viewpoint of aliens, both to criticize its typical writing but also to say 'okay, yes, it's naively-optimistic to an almost golden retriever-like level, but is it that wrong to hope for something better?' A big factor was, y'know, distancing them from a lot of the Obviously Jewish Stereotype bullshit, and instead patterning them on 90s commercialist pyramid scheme bros. But more than that, they were shunted to represent the ways that kind of mindset was a failure ideologically that has everything to do with systemic wrongs, not a failure of character.
-
Quark, who by all clear standards is a hypercapitalist, misogynist slimeball, is a failure in the eyes of his people not because of those qualities... but because he hasn't embraced those qualities enough. He's ultimately a somewhat tragic figure in this regard, because all he craves is the acceptance and love of his own people, but his own sense of ethics prevents him from living up to his religious belief in unswerving greed. Frankly, he borders on a satire of megachurch 'Prosperity Gospel' doctrine and how the only people rewarded by it are those so nakedly-sociopathic that they'd do anything just for a buck. Notice how many times he quotes the Rules of Acquisition in the manner one might say 'like the Good Book says', or him having an actual religious crisis including dreams of spiritual figures when he considers doing something his culture considers 'sinful'. He does everything the scripture commands, does his best to observe all the customs of the faith, follow every rule set out, and wallows in middle-tier irrelevance because he's already been singled out as, if not the first against the wall, just about fourth or fifth.
-
Rom has, initially, all the same failure modes Quark does. His conviction in capitalism is even more initially-pathetic because he is the first one up against the wall, and he knows his society's rules are that it's his brother's job to hold him down for it. He's doomed to exploitation unless he finds something else, and that pressure consistently pushes him towards deeper and deeper swings towards solidarity and good morals, because those pressures force him to rely on goodwill and the patience of others. He is put through empathy boot camp over the course of the series, and even then he has trouble shaking some of his older, worse beliefs. But he still ultimately comes around; he forms a union, he moves into civil service, he fights for the rights of women(albeit after fits and starts), eventually starts to try and turn his whole society around, and while he isn't by any stretch perfect he's reaching for something more than what he's told he 'deserves' by people who don't give a shit about him.
-
Nog is the biggest turnaround. From the disaffected son of a failing waiter being ruthlessly exploited by his own brother, to higher education, naval service, officership, and probably someday command of a starship, Nog is meant to demonstrate not just how truly valuable it is to believe in a greater ideal, but also how stupid it is to automatically suspect ulterior motives of cultural outsiders. Nog manages to harpoon the Federation's untrustworthiness of those who exist outside it by turning out to be the most absolutely gung-ho Starfleet officer anyone has ever met; but he's also doing so because he sees how utterly dead-end the hypercapitalist dystopia he comes from unless it can change. But he also keeps the things that work about his culture, the ideas and concepts that shaped what they became and shows they didn't need to become exploitative(see: his more philosophical breakdown of Ferengi religion in "Treachery, Faith, & The Great River").
Am I going to say the Ferengi were fully 'fixed'? Good god no; the Ferengi whiffed hard on concepts of gender stereotyping even into DS9 due to still wallowing in Rick Berman's unbelievable failures as a television executive and human being. You have to just kind of accept that you'll have to hold your nose if 'Ferengi' and 'gender' are in the same room in 90% of instances. But if I were giving a 'most improved' award for anything introduced in TNG, it would be the usage of the Ferengi.
I can't stress enough how much this aligns with why I actually really like the Ferengi in DS9, and why I think they serve an important purpose in the narrative weave of the show as it develops. Being culturally alien to the sometimes deeply twee Federation gives them a viewpoint that the writers manage to use to devastating effect once or twice - Quark's "Root Beer" and "Take Away Their Creature Comforts" come to mind.
Quark as an oddly tragic character is something I dearly love in how he's written. By the metric of most any other species, he's got it made; a successful bar with a diverse clientele he likes meeting and interacting with; friends and family that care about him; colleagues who (depending on who's writing the episode) respect him and his position, seeing him as a vital fixture in DS9's melting pot culture. But because he's Ferengi, it's not enough. What's telling is that he could be a better Ferengi, he's so close to profit and greatness all the time... but he'll never truly, properly exploit those on his path to it. He's a bad Ferengi because he could be a great Ferengi, but ew! All those morals!
There's a line in the show where someone mentions him selling medical supplies to refugees at cost, and while it's played in the script as kind of a joke, I think there's so much Quark explained in that single line of dialogue it's one I always come back to when thinking of him.