Recently came across of Q&A stream where Matt discussed many interesting things, one of which is inventory problem. In my understanding( they did not go into much details ) if they resolve problem of scrolling items as changing the look of the UI, its gonna help a lot, but still wouldnt be as ideal for those of us who have too much.
So my idea is not to reinvent a wheel, but implement something so common to MMO genre, a bank.
In the " lower left " area of the social where most people handout are 2 areas for clothing customization, always wondered why do we even need duplicate while a single one is more then enough. So my idea is leave Prestigue room for how it is ( because its bigger ) and change red room next to it ( forgot the name...derp ) to a bank area.
Now we have two options
1) Boring one
Just change visuals of the room and change functionality of machines inside. Different types of banks would be in tabs.
2) In-depth one
Put there 3 new NPCs( Could be made by you or start an even where people would submit their versions of workers of Bank of San Paro )
1. Personal storage unit - gives you ability to store " untradable " items such as permanent guns and other JT,ARMAS stuff.
2. Account bank - gives you ability to store legendary guns, cash, symbols, music and other stuff that are " tradable ", so you would not need to send 10-20 mails with a single item to your character back and forth.
3. Clan Vault - here you can deposit and withdraw basically anything that can be traded. Could be tied to clan system expansion, so some stuff can be taken only if you have specific role or " points " ( I will talk about this idea more in my next topic about clan upgrade ).
Conclusion
In my opinion banks are easiest way to solve inventory overload problem, also you can always sale " bank slot expansion " in armas, *wink-wink*.