The ability to buy additional character slots

 "As a non-subscriber, I want to be able to permanently buy access to additional character slots".

Which I think is a pretty simple and solid idea. By all means price it so it's still advantageous to purchase a subscription, but for users where regular payments might be unwise or who prefer to be up front on their purchases, the option would be powerful.

"Puts on the Hat of Solutions." - The difficulty likely comes from introducing a new field in a user's profile document so that dndb can populate the correct number of slots. Divorcing the slot limit logic from subscription tier.

"Puts on the Hat of Product Owner." - ...It might generate revenue and if we price it right it wouldn't directly compete with our subscription prospects...