Section 31, the secretive illegitimate covert arm of Starfleet, had Sloan who really seemed like he’d done a lot in the name of the federation. He tells Bashir as much in Extreme Measures so it would be really cool to see some of the darker things the Federation disavowed or turned a blind eye to.
Very different tone to most trek probably but a whole different side of the federation, more of the unhappy side like the Maquis who don’t get explored much either.