the only way I see a 3rd finisher working is if it plays off of shadowbringers light/dark theme,

that way you have a light finisher, verholy, biased on light mana
dark finisher, verflare, biased on dark mana
and lastly a null/void finisher unlocked I would say after a verholy and verflare then with even bars on mana, call it ver...i dunno, ultima

boom, a simple addition without butchering the job with dots or other nonsense, add in trait boosters, jolt III and an aoe finisher for even mana bars and rdm is set for another 2yrs