Idea for a new item: Vault
I have a suggestion for a new item, and I would absolutely pay for this....
A personal storage vault that is set up on the ground, without decay.
HEAR ME OUT HERE...
So, like, have something that is located on the ground (fixed to one location) but only accessible by you. You can store 100-500 things in it (depending on how much money you've put into it, and making it upgradable is good so we can pay for space a bit at a time as needed). You can only access the stuff in the vault if you're standing next to it, but once in the vault, it is out of your inventory space. This way it's not paying to have a backpack account to break the game, but it is allowing for customizing your game play inventory for the day.
The vault would be able to be moved via a "move the vault to this location", but if you do move it, you lose everything in it -- so to do a clean move, you'd need to empty it first. There should also be a way to navigate to it in case you forget where you put it.
The thing is that this is something I can store gear in for anomalies when I'm not at them, or store non-anomaly gear when I'm going to anomalies (no more pestering my barely-playing Ingress friends!) It could also help people defend their home portals by having extra inventory at access at that home portal, which does make killing a home portal more difficult but that is a cool game change I think. I would suggest to keep it from being game-breaking, limiting the vault capacity to a maximum of 500. But you could either sell them at 100 capacity vaults each for a max of 5, or have an upgradeable vault in increments of 100.
And this is mostly stuff that's already mechanically present in the game, so could be implemented fairly trivially. And it's something most Ingress players would pay for.