In the spirit of Halo characters being redesigned slightly every game, I took my own shot of the concept. Most of these designs are meant to be aesthetic middle grounds of multiple pre-existing designs, with some personal additions.
The Sangheili (Elites) are heavily modeled off of their H2:A and H4-5 iterations, rationalizing the former as a "gracile" morph and the latter as a "robust" morph, with some additional tweaks to make their respective anatomies at least marginally more consistent. The leg proportions of both are based on the H:R Elites, albeit with the floppy toes being substituted with the more classical hooves. In regards to the oral anatomy, I did two different, VERY mutually exclusive versions to choose from; a H2A-styled mouth with lips reminiscent of "The Duel", and a more Halo: Reach inspired mouth redesigned to be more anatomically feasable. For both versions, the elites would talk with the lower pair of mandibles functioning like a single lower jaw. The Covenant Elite combat harness is heavily modeled off of the H2:A design, though the mandible guards have been replaced with a face plate formed from four spring-loaded flaps, emulating the sort of seamlessly-closing mouth of the Combat evolved Elites. The Storm Elite combat harness, however, is essentially a Bungiefied version of the harness seen in-game.
The Unggoy (Grunt) is of course supposed to be modeled off of the traditional Bungie Grunts (or "Kafv" as they are called in-universe), but I also wanted to rectify the clash between the more arthropodal-looking Bungie Grunts and the more reptile-like Grunts of Halo 4. I figured that a good way to do so would be to depict the exoskeletal plates as being formed from enlarged and thickened osteoderms, similar to a turtle's shell. I also gave them lips, simply because it would make more sense for them to have lips since they are described as being capable of learning and speaking English and other human languages quite fluently. The armor is based off of the Bungie era combat harness, with the armored methane tank being a composite of the tetrahedron-shaped tank of the first three games and the more rounded arch-shaped tanks seen in Reach onward.
The Kig-Yar (Jackal) is based on the Ruuhtian (aka "classic") subspecies. Having recieved the most drastic redesigns of any of the Covenant units even throughout the Bungie era, this design is decidedly a chimera of the Ruuhtian designs seen in Combat Evolved, Halo 2 and Halo 3, with some additional design elements of real-world theropod dinosaurs thrown into the mix. The body frame is similar to that of the H3 Jackals, though I gave it more dinosaurian hands and feet, and the stiff quills are substituted by hair-like downy feathers (I like the idea of these guys perhaps styling their "hair" for the sake of individuality). The head is sort of a strange unity of all three Jackal designs, with an elongated H3-styled beak, H2-styled external tympanum, and H:CE-styled double-nostrils. Being canonically omnivores, I decided to give this Jackal heterodont dentition, with pointier teeth towards the front and grinding batteries of teeth towards the back, resembling a more omnivorous version of the teeth possessed by pachycephalosaurs. The armor is most heavily based on that worn by the H3 and H:R Jackals, with the addition of large green goggles that pay homage to the comically large insect-like eyes of the Jackals in Combat Evolved.
The most drastic designs of the Jiralhanae (Brutes) are often in regards to the head, so I figured this redesign would focus on the head specifically. I was aiming to create a compromise between the ape-bear-rhino look of the classic Brutes and the more orcish aesthetic of the Reach and Infinite designs. The skull is sort of an anatomical composite of ursine and rhinoceros, with the lips and nose of an ape and the thickened skin of a rhinoceros. A little design feature I've seen often get neglected are the keratin brow hornlets of the Reach Brutes, so I included those in this design as well. The inconsistency of protruding vs non-protruding teeth is rationalized as an orthodontic procedure; the deliberate surgical misalignment of teeth to appear more intimidating to foes.
Halo ( c ) Bungie, 343 Studios, Microsoft