Armour cores currently have no function but to gatekeep which armour pieces and colours you can use together. This is a bad thing, as it ultimately limits player expression. So, I have the following idea to make armour cores useful.
Armour cores could be used to further expand the different body types currently in game. This would lead to more player expression, as the player can choose an armour core that fits with the body type and style the player wants. The Reach Mark V [B] armour core could be a Halo Reach like body type for the Spartans, the current Mark VII core could be the currently available three body types. In the future, they could also add a MJOLNIR Gen 2 type core which would correspond to the body types of the Halo 5 Spartans.
This way, having different cores makes sense as there is a meaningful difference between them. The goal should be to have as much cross core compatibility as possible. If this is not possible, then at least it would make more sense due to the body types being different from each other making the armour incompatible. Ultimately, the goal should be to increase the number of ways the player can express themselves so that there will truly be a million different ways a spartan can look.