I would have never made him the final boss. Mannie into Archie, with Gul'dan escaping, is fine as a lead in to Legion. Grom is, at best, a intro "last 3" boss. He would have been the penultimate "Warlord" boss, but not the final boss of Warlords to lead into real stakes for Legion. Gul'dan HAD to live to give us someone to chase.I do like the idea of him being swapped with Fel Lord, but I'm not a fan of the mechanics you suggested.