In a word: Banks.
Each region has its own banking system. Banks become built into citadels and stations, with it being a fueled service on citadels.
Banks in HS charge 2.00% annually, LS is 1.00% annually, citadels an owner-defined % ranging from 0.00% to 2.00% (2 decimal places of precision so that you could charge 0.01% if you wanted to), paid the first of every month. In order to access markets or contracts (anything that ends up in a transaction fee currently), you need isk in a bank within the region.
Transfers between structures can be initiated for a modest fee of 0.1% within the same region, or 0.3% to a different region. Or you can fly it there for free yourself.
Isk can transported via encrypted wallet files, associated with your pod. It isn’t lootable (edit for clarity: but is destroyed) because it’s encrypted and thus useless to any attackers. If you want to carry your isk in your pod, there’s obviously no banking fees for it.
You don’t need docking access to get your isk out, but you do need it to deposit (one must dock). You may not initiate a transfer to a citadel you cannot dock at. The service only needs to be fueled to deposit isk, it can be retrieved or spent in low power mode.
In the event that someone destroys the bank that your isk was in (or it is unanchored), it is treated the same as assets in asset safety. In all cases, a pilot with a deposit will receive notification of every event related to this, so you can make informed decisions about where to put your isk.
The owning corp of the citadel gets 5% of the fees collected. Isk deposit totals are part of the publicly available information on that structure. The interest rate is set once when the structure is anchored and cannot be changed.
Why I think this adds value to the game:
- It’s an isk sink that has no meaningful impact on people with little wealth. Even at a modest 50 billion liquid wealth, you’re paying a mere 83m isk per month to store your isk in a HS station. Storing your isk at a low interest citadel obviously reduces this cost with an appropriate change in risk.
- It adds more incentive for structure fights when economic warfare against an actual rival is your goal.
- It adds incentive to try to attract investment in your citadel
The biggest potential detractor I can see:
- Players who take a break will potentially end up with all their isk in asset safety. This can be mitigated by offering an asset safety reduction of 5% for every month that your pilot is not training. It could also be mitigated by leaving your pilot in HS, with their isk in their pod.
Thoughts? Good for the health of the game? Bad for the health of the game?