It'd be great if on top of the Chief-Locke duality in Halo 5, there was a third reveal in the style of Halo 2 that an Arbiter campaign also became available.
Honestly, I'm seeing a ton of potential in Locke - my main concern is that they're going to make Locke's playable sections feel the most "modern" (aim down sights, using cover, blood on the screen, etc.) when in reality he's got a perfect setup to go in the complete opposite direction. Have Chief's campaign feel classic Halo, make Arbiter's a CQC-fest with a lot of "think, don't shoot" lore elements thrown in, and pit Locke against a bunch of weird enemies like the Prometheans and neo-Flood that force him to dig into his library of crazy new prototypical ONI weaponry. I still want to see an in-game implementation of the Hard Sound Rifle and that Gauss Sniper, dammit.