So I love playable elites in halo and it has been a staple since the literal second game in the series. But for some reason 343 seems to be dragging their feet on this so here are a couple ways I think that could work for adding elites:
First, a new armor core for elites only would be needed, possibly being added like the reach armor was in the first battle pass, as one of the first things you get for the premium pass.
Next, that battle pass needs to be 80% or more elite armor with next to none for Spartans (with the exception of that one armor set from halo 5 which I really like and I think would tie perfectly into the elite customization)
The best way I can see this working is if the Spartan armor cores are all combined and you can mix and match between them without borders, with cores only being used to separate between species, not armor generations.
This pass could also be tied into the campaign! Imagine you download a new campaign expansion and you don’t know what to expect, and while you are playing, you receive some staticky messages from somewhere, and at some point, you see a giant banished ship in the distance get obliterated by a huge armada of ships, and over the comms you hear “This is the Shadow of Intent, are you reading me 117?” Then as you progress through this new expansion, you get to have Arbiter as Player 2 in the coop and for Players 3 and 4, you could have either a multiplayer customized elite or Spartan and those players get to choose which it is!
Obviously this would be a giant undertaking for 343 but I really think that it would be spectacular to see done well. This story expansion could even give a reason to have Spartans and elites fighting side by side in multiplayer.