Elites being in MP is always going to be a tough one.
On one hand, you want a balanced multiplayer playing field and you can execute that perfectly when everyone is using the same player model with the same hitboxes.
On the other hands, you have those who prefer playing as an Elite, use Elite models for machinimas, Elite specific custom games and most importantly - mixed race gametypes like Invasion.
If you want to introduce Elites into Multiplayer, you need to make them have the exact same hitboxes and traits as a Spartan - something Halo 3 tried and failed. You ended up with Elites which were shrunken down from their actual height canonically, and had spots in their player model that could be shot through (specifically their neck).
Those same models were used in the Campaign and obviously didn’t compare to what Elites should actually look like.
Fast forward to Halo: Reach and Bungie introduced Elites that matched what they should be - Aliens which were tougher, taller and faster than Spartans. Issue with this was - larger hitboxes, stronger shields and faster movement.
For cross-species gametypes, it works perfect i.e. Invasion.
For gametypes in Matchmaking outside of Invasion, you played as a set species by Bungie. For a majority of the time, you were playing as a Spartan - minus Elite Slayer, Dino Blasters, etc.
If you truly want Elites in Multiplayer, then canonically they need to be stronger, faster and obviously have larger hitboxes.
Now, the issue you run into is - do we make a smaller Elite model for the Multiplayer which matches the Spartan model in terms of height, movement and hitbox size but confuses the player when they look at the Elites in Campaign which are WILL be taller, move faster and have a larger hitbox size?
It’s a tough situation. We have no idea what 343’s plans were for Halo 4 because they never made the cut. It should be an open discussion with the community to be perfectly honest once talks about the upcoming beta has begun.