You can face all of the gym leaders you fought before(excluding Opal, as Bede now takes her place), Marnie, Hop, Leon, and 15 new trainers named Yue, Polaire, Kent, Vega, Cher, Deneb, Icla, Wei, Izar, Dunne, Pia, Corvin, Terry, Theemin, and Phoebus
Source: https://www.serebii.net/swordshield/championcuprematch.shtml