My suspicion is what they've already put out there: getting to milestones in Classic will give you stuff for Retail. They could even do a "this will reboot yearly" feature and make it like doing prestige ranks in a shooter. Hit "Veteran" rank 1 during a year and you get X mount for retail.That’s the big unanswered question, what are the long term plans. I’m guessing it really hinges on how popular it is, and so far it has well exceeded what blizzard has thought it would do.
They have a few options. Leave it as is and let it be classic. Add content for more things to do. Or continue on with the progression of TBC, and possibly WotLK.
They all have their down sides, but for now people are enjoying it for what it is.