Romulans after TNG season 1. Thankfully, DS9 found a way to save them.
The Ferengi are a warp-capable species in the world of Star Trek, known mainly for their avarice and cowardice. The Ferengi became sort of joke characters in Star Trek: The Next Generation, used for gags and (mainly) light-hearted episodes. The money-hungry species would occasionally return to menace Picard and friends over TNG's seven seasons, but they were never considered a serious threat but more of an annoyance.
The Ferengi would get a second chance at life on Star Trek: Deep Space Nine. Rather than generic schemers, the Ferengi were made into fully realized individual characters. Quark, the station's bartender, is the prototypical Ferengi, outwardly obsessed with profit but with a surprisingly open heart and mind. His brother, Rom, started his arc as the stereotypical silly Ferengi before evolving into a valued member of the station's crew and something of a technological genius. Perhaps most progressive of all was Nog, Quark's young nephew who befriends Jake Sisko and eventually becomes the first Ferengi to enter Starfleet.
Deep Space Nine also went to great lengths to establish Ferengi culture beyond the limited ideals of conquest and profit as set up on TNG. The Ferengi were essentially hyper-capitalists, with the acquisition of wealth (and the Rules of Acquisition) their defining drive. There's more than a little overlap with the Ferengi and 21st-century human society, a connection Deep Space Nine was always eager to explore. The show also established the importance of family to the Ferengi, on par with the value humanity puts into their families, which went a long way toward making the Ferengi relatable and even likable.
The Ferengi are still almost entirely defined by their time on Star Trek: Deep Space Nine. They've made no significant appearances in Star Trek fiction since that show ended - give or take a brief cameo in Star Trek: Lower Decks. They're a race Star Trek could potentially have a lot to say about as the generation that grew up with Deep Space Nine grapples with the realities of late capitalism and economic hardship. Perhaps a further evolution of the race is in order, but it would take a bold new direction to equal the greatness they achieved on Deep Space Nine.