I have no idea how they plan to do his accessories but I’m really hoping somehow you can change his entire body armor not just small pieces. This way they could have actual accessory sets that change the look of the character completely, instead of just tacking on random bits that don’t really fit his overall look. Arbiter’s armor has always been in complete sets, trying to mix and match on it might look pretty bad.
Some things I’d like to see:
Default (Halo 5 Armor set)
http://www.halopedia.org/images/3/39/H5G-Thelrender.png
Other ideas
Arbiter’s pre-Halo 2 armor (Standard Elite armor)
Elite Honor Guard set
http://static.giantbomb.com/uploads/original/4/48290/2669702-gamescom-2014-halo-2-anniversary-first-person-delta-halo-you-dare-d0b22d438a88442f991914bb4fd20c72.jpg
Heretic set