With the light being set up as the next big villain, the Man'ari Eredar could play a great role and lead to some awesome lore for the Horde. Their storyline could involve a new leader emerging (Kil'jaden's son/daughter?) and leading them towards a life free of the fel, something the Orcs (Fel), Blood Elves (Mana Addiction), Nightborne (Nightwell) and Forsaken (Lich King) could relate to. It could involve a group of Man'ari that were against following the Legion, but had no out, and now that the Legion is defeated those individuals need allies.
They would be rejected by Velen, and hunted by the Lightforged as heretics, and would turn to the Horde for help in the face of extinction. Blizzard can use them to introduce new methods and powers to defeat the light from the Horde side, while the Void is used by the alliance side. It would add even more intrigue in the Horde, as the Orcs and Mag'har would be initially against it, while the Tauren and Nightborne and would want to give them a chance at redemption.