In Destiny all the enemies look the same. One Vandal looks identical to every other vandal.
This makes it feel like you're fighting hundreds of clones. So what I think would be nice is to have variable enemy appearances. Fallout 4, for example, has variable armour sets on it's raiders, with distinct helmets, chestpieces, shoulderpads, leggings, that can be combined in various ways which make the gangs look like they are composed of individuals, instead of duplicates. I'd really like to see a system like that in destiny, make everything look distinct. Obviously there would be overlap in the combinations,
This sort of system can also help ensure that all the boss enemies have a unique preset appearance, instead of reused models.
It makes sense from a story perspective too.
The Fallen are ragtag, and most of their gear is salvaged and old. They should have tons of variable armour designs, with modifications and repairs. They aren't the sort to have a uniform. They also strike me as the type that would pride themselves on their unique armour, adorning it with shiny trinkets and patterns. It's probably the only thing a lot of them own.
The Hive have biological armour, that would probably grow in distinct ways for the individuals. While a thrall with a different set may not look too different, the Knights, Wizards and Ogres would probably have quite impressive and unique Bone Carapaces.
The Vex are uniform, but the system being applied to the other enemies would make the vex and their identical legions stand out more.
The Cabal have a uniform, but soldiers on long deployments have been known to paint decals on their gear. The Cabal may also have battle damage on a lot of their armour, that their command hasn't repaired do to their lack of concern about individual soldiers.