I missed this cool feeling from World of Warcraft. My account was created on the day of release in 2004, so sure, this whole idea is a little self-serving -- I would be eligible for any system devised for the greatest rewards. Could I have just had an epiphany? If I make the rules, I can totally reap the best benefits.
Two options for loyalty rewards
There are two types of loyalty reward programs that I think could work well in the WoW environment. Each program looks at when the player's account was created and, based on that, doles out account-bound rewards in a limited fashion. The fun part is that a loyalty program could pull older players back into the game because of their pending cool stuff.
One option could be to create one gift for every year the game has been in operation and give each player the gifts that they are eligible for. If little Timmy started playing WoW in 2007, he would get something fun in his mailbox for each year from 2007 to 2012. That's six fun things! Transmog items, mounts, pets, titles, and more could be granted or given based on the number of years the account has been around.
Another option could be to give each account tokens based on the number of years active or since created and let the player choose which rewards that they would like to have. Back in the Ultima Online days, I remember that I got to choose a loyalty reward or two, and that choice, while final, still gave me a good feeling of a well-made decision. You had credits added to your account at each milestone mark and could get any of the gifts, past or present, but were limited by your tokens. With this system, the longer ago you created your account the more tokens you get, but the rewards available for everyone is from the same pool. If you started playing in 2010, you only got two tokens but you still choose which rewards you want out of what everyone can choose from.
Loyalty program overdrive
How do you kick it up another notch? What about making loyalty rewards account-bound after use but tradeable when they are first created? Players who don't care about mounts, pets, titles, and anything else that the loyalty program could provide would instead be allowed to sell their items much like how players sell valor and justice items that are bind on equip. Not only do you reward players for staying subscribed, reward ex-pats for coming back to WoW with a way to make some gold on the Auction House or fly around on a cool dragon, and new players want to get on board to start earning their yearly credits.
You wouldn't even need to make 100% new items for the loyalty program. What about if one of the mount options was the Onyxian Drake for two loyalty tokens or the Blue Proto-drake for one loyalty token? These are mounts that players have farmed for years and never gotten because of bad luck and are already random chance mounts in tank/healer satchels (except no Onyxian Drake in those bags; it was just an example.) Now, with a loyalty program, you give these players some luck amnesty, let them fly around on the mount of their choosing from the program, and retain players for even more years to come. While you wouldn't give away mounts or rewards that were too rare or purposefully removed from the game, you could definitely make a lot of players happy with some luck amnesty.
A new currency
If Blizzard decided to create a token loyalty program, this would mean creating a new currency that Blizzard could use as a multi-use reward chip. Great post that made it to the front page of the official blog? Your account has been credited a veteran token. When Blizzard throws a random screenshot contest, community managers can easily dole out veteran tokens based on a set of guidelines built into the system, since everyone posts on the forums from their accounts anyway. The best part about it is Blizzard has total control over the reward distribution and its own currency, allowing players to get tangible rewards in game for participation outside. Why not?
If it's not already apparent, the free Diablo III promotion combined with the WoW Annual Pass helped retain subscribers and was ridiculously successful, with a million accounts locked in for a year. Lock players in more with another loyalty program that requires them to do nothing but spend tokens they've accumulated by just hanging around. A WoW loyalty or veteran program could take many shapes and forms, but at the heart of the system is a way to bring back and reward the old, put another hope in a new player's heart, and provide fun rewards that facilitate trade and good attitudes. I think it could work well.