yeah no idea, I kinda doubt that he'll get a Mark 7, at least they shouldn't do that, he's just come back to earth at the end of Halo2 from a really short trip so I mean it'd be kinda silly if they were like "well while we were being invaded we made a whole new suit" now it'd be cool if they just maybe had a couple add ons or adjustements they could do which could be feesable. But I see it as he was out on Reach and away from Earth for a long time from when he had the Mark V which was actually brand new and experimental in the original Halo. He got it right before they left Reach and he was the first Spartan to get it so yeah. But heck who knows they probably will give him a new suit in Halo 3 just to mess with us and show off the next xbox's power or whatever.
Anyways I honestly hope the Arbiter goes the way of Raiden and that we only see him in cut scenes to move the story along. I dont want to play as him anymore, I was tired of it after 1 level in Halo2 and really pissed he was the last one you play as, it was a neat thing at first but that one level should have been it. Just like with MGS2 I bought Halo2 to play as Master Chief (or in the latter game I wanted to be Solid Snake) I dont like being thrown in as some sidekick lol