The basic idea. Make DDI a multi edtion platoform but have D&DN as the flagship of course. Leave the 4th ed material up even if it is on an options menu somewhere.
Have some sort of "in game" currrency I would suggest the gold piece. You use this to buy subscriptions or whatever they can think of selling. Devote 20 pages of Dungeon magazine to "Classic" D&D which is everything that is not D&DN. Or create a "Dungeon Classic" spinoff for the older stuff. Dungeon Classics is purely fanbased and is reliant on players actually creating material for it and they are paid in "gold pieces" perhaps even a large sum of it ($100 worth of virtual currency or whatever).
Let the fans decide to some exent what they want and will actually pay for. IN the classics section they can also develop new material such as maybe a 32 page AD&D module or BECM , 3rd and 4th ed adventure maybe even going as far as letting the fans write it as well or they can pay a professional. Try it out and make it a micro transaction for $5 or $10. Something cheap anyway. Think about how well a new AD&D adventure or module would likely sell. Also consider putting up PDFs of the 3.5 core rules as well and maybe the AD&D (1st and Second ed) so for you Yearly/monthly fee or whatever you basically get 3.5, 4th, and D&DN material and older material as well. Some of the older material can actually be new. The D&D Rules Cyclopedia could also be put up and that is one PDF that would sell well I would imagine if it was sold on RPGnow.com.
If you feel like a break for your edition of choice you can switch to another one for a bit. The overall idea is to get as many D&D fans to give you money and let the fans to an exent publish their own work. The cost to WoTC is that they would need an editior for the Dungeon or Dungeon Classics PDFs and artists etc which they already have I suppose. Frebies could also be handed out on occaison to subscribers as I recently got an email from Paradox Interactive which gave me a free copy of EU III Chronicles on Steam as a reward for thier forums reaching 500 people. I had actually registered my games I bought off them instead of pirating them and the games have been cheap on Steam anyway ($2-$15 on sale).
Just an idea.