Well there are no explanations why 2 styles of aliens decided to join forces. My guess the classic elites (Originally the "storm Covenant" but with classic armor) belong to the Harbinger and the Banished are Banished. If you think these designs don't make sense well then you can say the same about Halo Wars 2.
Agree with you.
All these "redesigns" and actual new sub species, phenotypes through the franchises games/ other media could all exist together, for varity for also to be intervowen and explained canonicaly, lore wise. It might be a big effort but REALLY worth it in the long run to clean upp loose ends and mess.
I know the one part in Halo 5 where the Arbiter had the new sangheili/ elite looks were a doozy bummer. But sure got corrected in H2A new cutscenes. So kudos for that.
As I see it. The jiralhanae brutes have at least 3 subspecies. the ones from H2A, the H3 ones and the Halo Wars 2/ Infinite ones. Some are more savage, some are overall bigger, stronger and some are more agile. All could play role gameplay wise.
Also the Kig Yar. The Jackals have two sub species and the Skirmishers one.
Grunts, or unggoy migth have 3 subspecies too. The one from Halo 3, 4/ 5 and Infinite.
The Elite sangheili seems to have several subspecies or phenotypes at least as well.
The classic, darker grey skinned ones from Halo 2 A, the Halo 3 ones but those are more brown skinned so in comparison to H2A ones those are for each others phenotypes. Then you have the very agressive and nimbler elites form Halo reach which have lighter greyish skin, darker eyes ( more of an oversight ) and then we have the halo 4/ 5 elites which seems more stronger, kinda dumber?.
Another phenotype or even possible a phenotype is the Elites from Halo Wars 1, those seemed taller, slimmer waists and also a different setup for their head, mandibles etc.
In fact all the looks ever seen can be explained, intervowen and made clear for lore/ canon sake with reasons of roles, where on their homeplanet/ moons ( s ) and home systems ( s ) they evolved.
Like the Skirmishers, the T'vao orignated from the Kig Yar homeworld of Eayn but settled outside of their homeplanet and evolved much more differently than their fellow Ibie'shan ( halo 4/ 5 ) and Ruuthians ( standard jackals ).
they made a good job of explaining those, so why not the other races? Im sure I might miss some infobits so feel free to give input.
Also they could update all those subspecies, races i ngenral and phenotypes with next gen graphics for full glory... eventually.