Battle Dress in the Imperium is complex and generally misunderstood. If you were designing Battle Dress and it didn't currently exist in the Third Imperium what capabilities would it have? Would there be integral weapons? Electronic Warfare? Would the exoskeleton add strength or just endurance? Inquiring minds want to know.
There is something to be said about non combat focused accessories. Water reclamation & filtration system. Air purification, I can see a vacuum rated suit that has the air storage and CO2 scrubbing as an installable module to reduce weight in breathable atmosphere missions. Jetpacks. Battery pack, with standard connectors so not only it powers the suit for an extended time, it can also power your energy weapons and tools. A grappling hook/winch.
And, not an accessory, but hook points all around the suit to haul crap around in satchels and the like. Logistics is paramount for war.