I'm just going to touch base with my own opinions, although most of it is a simple rehashing of what Kitchen Sink and Vholes have said, honestly.
Pay to Play: Agree with you here. Remember an old website that went from beta to Pay to Play… and lost almost all its player-base, because there really wasn't anything big/fancy enough to warrant the fee.
Micropayments: This does create a bigger gap than most of the other forms of donation, even when the items are tradable. Usually seen in the glut of recent Asian quick-release mmos, I'm honestly surprised that more and more of them are springing up. It doesn't seem to be a survivable system because it gives repeated content to anyone, and makes the game 'easier', but only if you pay them. Can be especially frustrating if combined with grouping systems or the standard Warrior/Ranger/Mage/Item Finder classes that are becoming cliche. But that's another rant.
Ransom: This is an interesting theory, but I see it as less viable. It falls into the realm of everyone watching a car crash, but nobody having the presence of mind to call 911.
'Hmm, well, he only updates the total every day, I can wait until my next paycheck before donating. Maybe 2 paychecks, no hurry'. Plus, as you mention, creating content and not releasing it? At that point you could make a free/member game with fees like the Artix Entertainment Games often have. It's a model that can work if you keep expanding both parts, but increase Free elements a lot slower than Member elements. But it's tougher to pull off without a real 'crowd drawing' element.
Reverse Ransom: Another interesting theory, especially if you can present several roughs at a time, and players effectively 'vote' for their preferred rough by donating for it. But the time between starting and ending could be a turn off, unless a rough time could be presented. Voting period X days, production period roughly X days. As long as people have an idea of when they're getting their stuff, they usually don't mind so much, but there will always be people unhappy with a lack of instant gratification.
Kol System: A credit item, which by itself is useful, traded in at an in store shop. This is good, but also gives rise to the problem of the Haves and Havenots from micropayments. The primary difference in this case is that KoL tends to rely on a balancing system for maintaining availability of new content for all players (Bottles of Absinthe, Astral Mushrooms), even as the old content is removed. However, it is imperfect (Please don't smite me Jick, you know it's true!) Every now and then they drop in must-have items, usually by accident (like the Psycho Sweater), which by themselves 'break' a portion of the game, then have to backtrack, fix, or tweak elements to bring them in line. I think the Llama llama was the last instance of this. Or the Sticker Tome. The limited availability raises demand amongst players, but if you have sufficient in-game skill you can earn enough in-game money to usually afford a donation 'Token' to trade in for the item. This works because content is 'nearly' unlimited (However many people donate for) so everyone who wants one can usually get one, usually, but only for a limited period of time, creating a constant source of demand. The problem exists when items start to roll off the bottom of the list. The 'extinction' of 'wild' hand turkeys and Jill o' Lanterns for example. Another thing to keep in mind is power creep. People have the annoying habit of not wanting the same items again and again… and of course, they prefer things that are 'useful'. So over time you keep having this annoying balancing act between game play, game mechanics, accessibility, and player happiness. If you can pull it off, good job. If not… err… Tough break.
Mixed Bag: This is the one I find most appealing, although it may be more trouble to work through. Combining the donation elements of the Kol System with the 'Goal' system from the ransom system gives us instant gratification as well as long term goals, which may contribute to small increases. Gaia online has a similar system, although it's closer to classic KoL donations. You give them money, they give you a 'donation letter' which, around the middle of the month, can be traded for one the two items of that month. Once they finish developing it.
Really the only appeal I can see with this is 'limited month based content' in addition to a devblog with a little more information on what's in the works. That is, more information in addition to the donation goodies, but you do that now anyway. Of course, items specific to what's being worked on as an additional bonus (ie, Plaques, awards, a pet puppy which is a bad 'pet' but is only available if you donated to the Pet System fund) could work as additional incentives. You get item X for donating, which can be malled, you get item Y if you were the donator, you donate for goal Z.
I can't say any of the systems are outright perfect, but the Mixed bag option sounds good. You could try an 'Antique' system like Secret Society Wars had (not sure if they still have it.. haven't played in forever), where items phase out of the donation shop over a period of months, but increase in price. You could offer a pool of items, and slowly phase out specific ones at specific non-month intervals (Midgard Business suit, added March 18th, leaving May 15, Omnitech PDAAI 9.0 Added January 2nd, leaving TODAY). You could use the Legacy system Cardmaster Conflict offered for donations (Man, haven't played that in forever too), where last year's card became available for the month as well as the new card. Whatever you choose, just remember to consider the rest of the game and how the things interact. Playtest them, and always remember, you can never say 'The players would NEVER try to abuse it this way!'
And while I realize this isn't suggestions, the narrative for a 'Donation hub' got sort of stuck in my head.
In Mikhail's Diner a new face Arrives. Bob is a simple man. He owns his own truck, and he runs his own small scale company. With big players in town like Midgard, Omnitech, and Zaibutsu, there are usually some small companies trying to work their way in. They need help that isn't already paid off. That's where Bob comes in. Bob's honest, and works hard for his money. The problems come from when a small company starts making a product that the big boys take notice of. It can be anything from some new R&D, the company being merged into the Corporate collective, down to a barely close enough to count copyright lawsuit from the big boys. Seems the small companies are having a tough time of it these days. So is Bob. When the company goes down, there's no place for his shipments to go to. Bob has to make up the cost of the product, and that's where you come in. Help Bob shift some of this excess product, and maybe he can toss the money to some of the companies downtown to bring you more exciting stuff. Help Bob Help You.
Either way, I will be a supporter for Metro, because while it is still rather raw around the edges, it has a lot of promise, and seems to be very well set up to avoid outright 'overpowering'. Although I may end up eating those words.
