He is the likely neutral character to guide us into Shadowlands(a la Khadgar of WoD) with Vol'jin as the main horde character and maybe Daelin or Uther as the alliance one. Bolvar seems very anti-social to do a thing and likely he is planning to launch some massive offensive to the world and Bwonsamdi doesn't care about mortal affairs. Besides Medivh power to travel to realms is his main appeal and way to travel between the world of the dead and living.