For MMORPGs I generally like monthly subscription models. I suggest one teak to this model. Make it tiered based on customer loyalty.
Months Cost/month
1-3 $15
4-6 $13
7-12 $12
13-18 $11
19-24 $10
24+ $9
Include a 5% discount for 3 month lump subscription and 10% for six month to a year. If you invite a friend who subscribes for at least 3 months you move to the next tier. If you purchase an expansion you move to the next tier. If you are at the lowest rate then you get a month free for the above. If you stop subscribing and come back you would start and the previous tier.
This promotes two key pillars for a positive online experiences; core player base and growth.
My personal experience:
Thanks to my brother-in-law, my wife and I got into WOW and played for 6 years. For good or bad we got friends addicted to the game as well. Though we were loyal subscribers and fans after six years Blizzard really didn't return the love. $15 dollars/month and a $40 expansion every two years became a turn off.
Now I subscribe about 3-4 months a year, $60/year, and wait for the expansion's price to drop. If Blizzard had a tiered model I would always keep my subscription. Why not at $8/month. At the same time Blizzard would be getting $96/year.