What ho,
With the end of each expansion, the subscriber numbers drop significantly for about a year before picking up again with the arrival of new content. As Blizzard has shown, there is little possibility for them to push out bonus content during this period, as all resources are going into the new expansion. This might sound trivial, but it is a loss of about 200-600 million dollars in potential revenue.
However, there are other ways to keep players motivated to stay subscribed. Little perks for the loyal fans, if you will. Just enough to slightly tip the scale over in favour of staying subscribed, rather than giving it a rest for a few months. They did this with the Annual Pass a couple of years back, but I believe that the concept would be more successful if it worked on an accumulative basis, rather than encouraging people to bind themselves for a year at a time.
Here's a very rough example, just to give you the general gist of what I mean...
World of Warcraft Loyal Player Programme
Long-standing Subscriber Perks:
120 months (10 years) - Receive the coveted Exalted Champion's Ring for your faction of choice! This is a real-life item to wear on your finger - similar to the sword, shield, and helmet given to long-standing Blizzard developers.
100 months - Receive the digital versions of all future expansions of WoW for free!
80 months - Receive an exclusive in-game mount and battle-pet - this amazing pair is yet to be revealed.
60 months (5 years) - Receive the in-game transmogrifiable item-sets: Horde's Edge and Alliance's Bulwark, in Plate, Mail, Leather, and Cloth versions!
40 months - Receive a discount on your monthly subscription to World of Warcraft.
20 months - Become a Blizzard developer! Unlocks access to unique development polls, player feedback discussions, blogs, and more!
12 months - Celebrate your first year in Azeroth with free copies of the game, including the most recent expansion, for two of your friends!
6 months - Receive a free Character Boost to Level 90!
3 months - As a thank you for becoming a citizen of Azeroth, receive a free mount or battle-pet of choice from the Blizzard Shop!
Months Subscribed During Specific Expansions Perks:
20 months - Receive a care-package to prepare you for the next expansion! Includes potions, bandages, and a basic set of quest gear appropriate for and only usable in the next expansion.
16 months - Receive a new title with each expansion - first up is 'Veteran of Pandaria'!
12 months - Receive a 25% bonus to reputation gained with all factions!
6 months - Receive a 50% bonus to experience gained on all characters!
Again, these are simple examples to showcase the idea. The actual perks could differ wildly from these.