OK so let's look at the worst case, WOTC is an evil company, sinister Machiavellian scenario and see how its plays out.
Assumption: WOTC want to make every dollar they can off of Dungeons and Dragons in every way they can, and doesn't want any other person or company to make a dime off of D&D.
Obstacle: There is an OGL out there which gives 3pp's the ability to make products compatible with 3.x version of the game.
Solution: Create a new edition. Get the 3pp's the ability to make products for your new edition. Have a clause in the contract wherein they cannot ever make a similar product for the OGL, and if the new 4e product existed under the OGL, they cannot ever again make something in that product line again for the OGL. The phrase "product line" of course is to be determined by WOTC. (See section 6 of the GSL.) Encourage people to convert their material over to the 4e. Discontinue the GSL. 3pp's cannot go back to the GSL for any of their existing products which they newly made for the GSL or converted over from OGL. This will effectively put many of them out of business. (Section 11, GSL)
Obstacle: There is a revocable d20 license out there which gives 3pp's the ability to make products compatible with 3.x edition of the game.
Solution: Revoke the d20 license. (already done)
Obstacle: Lawsuits by individuals or 3pp's over terms of the 4e GSL.
Solution: Make it very difficult to due based on jurisictional limits (can only sue in Washington state, King County) and contract that the 3pp must give Hasbro Attorney Fees if they lose. (already done)
Obstacle: 3pp's do not convert from OGL to GSL, and continue to make 4e compatible products under the OGL.
Solution: Create a 4e which has so many different terms, and such a different structure of character and monster abilities and traits, that anything produced for 4e under the OGL by 3pp's will be so hard for a customer to convert, that it dissuades customers from buying those products.
Obstacle: A highly talended financially sound company (Paizo, and whoever produces Mutants and Masterminds, and any other company producing an OGL based game) creates its own game out of the OGL, and gives 3pp's a venue to publish products for. This also creates a competitor system, which may draw away consumer spending from 4e.
Solution: Lawsuits to try to drive them out of business. Whether legitimate or not, make it costly enough to defend by those 3pp's that it effectively drives them out of business, regardless of whether WOTC wins or loses on the merits.
Solution: Along with the lawsuits above slap injunctions on those companies above, as well as all companies/websites selling their products. An injunction will stop sales of the product until the lawsuit is finalized, which may take years. It is in Hasbro's best interests to make it take years to resolve, which is not very hard to do. This is done in order to ensure the 3pp's go out of business sooner due to lack of revenue, and talent having to seek other employment.
Solution: Name anyone who freelances or is in some way associated with said 3pp's in the principal lawsuits as co-defendants. They likely will not have the ability to defend themselves finalcially, and will have to settle out of court. The main condition of settlement is they they agree never to produce anything for any OGL based game, nor sell or distribute any such products. Make the terms of the settlement confidential so there is no chance of an antitrust claim brought. This is a very common type of settlement clause in exchange for not being held liable for monetary damages.
Obstacle: Some 3pp's are hesitant to sign on with the GSL, due to the one-sidedness of the GSL (termination or modification at any time, with no warning necessary, giving up control over your product line and for which system it may be produced, etc., which are alreadyl clauses in the 4e GSL)
Solution: Assure them changes are coming, get them to sign up to the current GSL. Drag out and delay the revised GSL for a long enough period that some of the smaller 3pp's have to sign up due to the fact that they need revenue. Never produce a 3pp-friendly GSL. By then it is to late, most of the 3pp's are already in bed with Hasbro and can't get out in a financially viable manner.
Obstacle: Fan sites. The new terminology of 4e is so distinctly different than previous editions that WOTC has a viable claim that they are trademarked. If gamers publish heir character sheets, modules, monsters, etc. which they use in their own games on websites or elsewhere on the Internet, there is a trademark dillution. If WOTc does not exercise control over the new trademarks, and try to shut these sites down, they can lose control and ownership of the trademarks entirely, in essence creating a 4e OGL by not doing anything.
Solution: Threats of lawsuits on the companies hosting the fan sites. The owners of the hosting companies have no idea if the caims are true or not, so in order to avoid being named in a costly lawsuit brought by a multi-billion dollar company, they shut down the fan sites.
Solution: Promise pissed of fans/consumers that a solution is coming by way of a Web site license, letting them post their materials on their web pages. Never deliver on that promise.
Solution: Instead of a web site fan license, direct all fan based material over to DDI, which eventually evolves into a facebook/myspace format for sharing said information. Declare that anything on officially sanctioned DDI pages is ok, because WOTC will put in all proper disclaimers on every page, letting everyone know who owns the rights to the trademarked terms. Added benefit to WOTC is that eventually they charge a subscription amount for the hosting of the documents, in addition to the standard DDI membership. People sign up in order to have a place to post their stuff, and in order to avoid being sued.
Obstacle: pdf file swapping and loss of revenue.
Solution: Move to a completely electronic model over time, wherein the books or supplements are completely integrated into the DDI system, and are not a distinct part separate from the actual electronic interface itself. Purchasing supplements eventually evolves into purchasing programming code, which gives your characters and DM's more options. There effectively is nothing to swap over the internet, since there are no longer any books to convert to pdf's.
Again that's the worst case scenario, which as a lawyer I would advise my clients of if they asked.
The client/business person then assigns a risk factor to each of the elements above, and decides if the risk is worth the potential reward.
Just my 2 cents...these are things that went thru my mind, but this is the first time I ever actually put them down on paper, so to speak.