I had created a post on my blog awhile back regarding something like this. Everquest has Veteran Rewards, little buffs or bonuses you get for every year of subscription you have with the game. I'd like to see something like this implemented in WoW, and I wouldn't be surprised if it was on the books at some point.
Here's an example of what I'm talking about. In Everquest, someone who has had a subscription for one year receives the abilily "Lesson of the Devoted". This gives players a double xp buff for 30 minutes, with a cooldown of 20 hours. I'm sure Blizzard could come up with (coughstealcough) ideas for their own rewards, but something like this would be a nice little nod to their long-time subscribers while not throwing things out of balance.
Maybe they'd simply go with a special title of some kind. Whatever. I'd prefer something that benefits my character rather than my ePeen, 'cause there's folks out there who have been playing since Beta and still couldn't find their ass with both hands and a map. My two copper on the subject.