Have you ever considered releasing the source code for this game?
Dungeons and Dragons is one of the most popular games among the hacker community, and by releasing the source code to Dungeons and Dragons Online, you would be offering an open invitation to those hackers to come by and develop and improve your game for you. This could inevitably result in ports for multiple operating systems, thereby greatly expanding your market and increasing your opportunity to profit, as well as far more stability, reliability, improved performance, features, and flexibility through the myriad of addons that would be available.
There are only so many of you, but there are legions of programmers all over the world who would love the ability to help develop DDO and, since you already release the client free-of-charge, the free release of the client and source code and a $5-a-month subscription fee for access to your servers should easily allow you to maintain your current profits, if not increase them, while allowing you access to a global development base of elite programmers who develop projects like this for fun.
Not only would this be far less expensive for the end-user (half the cost of a VIP subscription and access to far more dungeons, features, etc. than what is currently available), but it would also offer you a continuous stream of revenue from monthly subscription fees, allowing you to bring more exciting games to the world, with hardly any additional effort on your end.
In my personal opinion, the open-source model could only benefit the company and the users -- after all, look at what the open-source movement has done already and how quickly they have done it.