"Our terminals in Halo 2: Anniversary are going to focus on The Arbiter. You'll start to get an idea of his history and a little more about him."
-Ayoub, 343

Seems to be that he's a bit of s focus, so far we have Locke as a second playable character in Halo 5, surely we're not looking at 3 playable characters?

Although, I can't help but admit that it would make a very interesting campaign, and remove all the pointless grindy missions from previous Halos

I hope he returns as being playable.

Latest OXM confirmed that Locke would be enlisting Arbiter's aid for Halo 5 [as if we didn't have enough hints of that before!] and considering H2A terminals will delve into his backstory I see no reason for this level of buildup if Arbiter isn't going to show up in a major way.

But I REALLY don't think we'll get a 3rd playable character. 343 wants to make Locke an integral part of franchise fiction. Having both the Chief and Arbiter in there would just soften the impact of his debut game [and I think he'd be eclipsed in terms of a bada$$ery standpoint as well XD].