Big Pockets

[Kotlol]

Wallets I think could be nice. Just like a civilian pouch but for cash and pens and junk

Exactly stuff like this. 
Maybe just having things that work similiarly like "security pouches" could be nice , though THAT frees up the slot for Jetpacks and such.
I simply recommended a "new" backpack, that is something you can wear under the "Coat slot"

Since more often then not... sometimes backpacks stick out something that looks cool design wise, but gotta have that satchel/backpack blocking it or else you can't carry things. And not everyone can get their hands on utility belts (wich again frees up the jetpack option and such)

I just wanted a cosemetic item that functions like the backpack but can be "practically" invisible if you wear a coat and such.

Also Nukies satchels is indeed one of the ideas for it, but the difference with those is.. you don't wear them on your backpack slot.
This is litterly A BACKPACK SLOT!