I have no idea what Legacy would be about, though presumably if Seven's involved then it could feature stories about the Fenris Rangers, Janeway, Naomi Wildman, and the aftermath of the Borg's eradication, among other things. Like how Strange New Worlds has stories about Pike's future knowledge, Spock dealing with love, and M'Benga trying to save his daughter, and you wouldn't see any of them in Legacy.
TOS and TNG have the same premise on paper, but their Klingon episodes couldn't be more different, due to the time jump and the presence of Worf in the main cast. Now that the Romulan Empire has collapsed Legacy can stick a Romulan on the crew and explore that situation some more, or a Cardassian, or a Kazon. Strange New Worlds can't do any of this, just like Legacy can't do a story about veterans of Disco's Klingon War.