Sangheili in multiplayer should be different than spartans, like what they did with Halo Reach but better. they should also be in many more modes.
Personally I love what they did with Reach, and I know it's impossible to ask this because it would be so much work for very little payout (or maybe not?) but, in a perfect world, the playable elite option would have the ability to play as Elites that are greater in size and have different stats like in Reach, but only have those elites playable in Invasion. This would be great but imbalanced for core competitive multiplayer, so for
competitive multiplayer, they scale the elites down (or just ban them from competitive play, I don't know).
And for modes where it doesn't matter, you can set your preferred size to big or small for forge or custom games just for the hell of it. I mean I think it would be perfect since I know a lot of friends who like 3's format where Elites are just as big as spartans, but I myself and a lot of others too love towering over spartans.
So essentially the idea would be to please both audiences, while
you please the playable elites audience as a whole.
That's perfect world though, if they were going to balance time with money, they'd have to either pick the 3 format or the Reach format. It's sad that they'd have to split up the Elites fandom that way but it's just the way it is.
This is strictly size and how it affects multiplayer though, nothing to do with armour or assassinations (which would be hard to do with different sizes of elites). For assassinations, I really really loved the bigger elite on smaller spartan assassinations so I have to vote the Reach format.
And I also don't mind not using them in spartan competitive play. Invasion was my cup of tea anyway.