Honestly, the Federation itself. I always thought you could make a really interesting show that was a mix of Federation politics and Starfleet intrigue; maybe a show about the Federation government on Earth that delved into Federation society and dealt with an overarching plot. Like maybe a season where a Federation colony at the edge of Federation space gets into conflict with another race/government, and for a season you get a mix of stories from the colony, a Starfleet ship sent to the colony to support, and the Federation Council on Earth.

Federation politics could be really interesting, but the shows always paid kind of disappointing lip service to it. One key example is in the DS9 episode “Rapture” when Bajor is being accepted into the Federation; the fact that a Starfleet admiral rather than a civilian Federation representative was overseeing the proceeding was lazy to me. Plus the entire ceremony was tiny and anticlimactic (and obviously barely got started since Sisko interrupted). Great potential for some real insight into the government but instead it’s just some rando admiral.