I think that they will bring the Elites to multiplayer when they bring the Arbiter to the campaign, if they ever do. The Arbiter was Master Chief's Ally in Halo 3, you could also play as elites in Halo 3's multiplayer. If they bring The Arbiter back in Halo 5, there's a possibility the Elites will return to multiplayer. If they don't, then either 343i aren't bothered or there is a fair enough explanation to why they can't. I hope they are trying.