Kill Ten Rats authors have previously come out for selling RMT fluff, and have featured games that use that funding model. What games will sell you more character slots? Guild Wars does. Sword of the New World effectively does so, selling slots for in-game currency that you can buy with meatspace dollars.
For the player, this offers convenience and potential savings. All your characters are associated with one account, so you have one monthly fee, you need not re-buy the expansions, and any special account upgrades (RMT fluff, veteran rewards, etc.) apply to your new characters. Keep all your characters on the same global name in City of Heroes.
For the developer, this has some advantages over trying to sell second accounts. You have fewer of the problems that arise from people using multiple characters at once, many who will not but another account will pay a one-time upgrade fee (maybe multiple times), and you get that money now. Policing for discipline is easier when the player is not a hydra. The only ongoing cost is storing more characters per account (perhaps not insubstantial).
Let’s say $5 for one additional slot per server, and you must buy at least four at a time to prevent transaction costs from eating the money. Alternately, you could have several subscription options, some of which offer more character slots or other premium services, but you would need to decide what to do if a guy with a 16-character account wants to cut back to a non-premium account.