There's a spectrum between Halo's current mission-based design and true open world play. I'm not expecting something too far along that scale. ODST moved a bit in that direction, but not fully, with a hub world that started off small in scope and then opened up, with traditional Halo missions branched from them. Deus Ex is similar, with a pair of hubs that you depart for the main missions from.
So my hope is less for a full open world Halo game, than for a game that moves the series in that direction, either building deeper exploration elements into the missions themselves, or into the game structure.
Halo's gameplay combined with that concept art would just fit so well together in a context that is more open than ten separate, linear missions in a row.