The hype and success of Classic can not be put in question. At first glance the original projections being thrown around were 250k players, maybe 500k players, however, the numbers landed somewhere between 1.5-2m.
Blizzard has unlocked an outstanding additional revenue stream but more importantly opened the doors for future directions which don't conform the linear progression path of expansion after expansion.
So, what's to come from here on?
Is it Classic+, is it TBC, Wotlk, classic server resets?
Yes, yes, yes & yes is the answer.
TBC is an easy cash cow, 100% chance of happening.
Wotlk is an easy cash cow, 100% chance of happening.
Where Classic+ comes in is it's an alternative path for those that don't want TBC but want something more then just Classic. The Classic community and the #nochanges movement are extremely sensitive and TBC divides the current Classic player base.
Classic has a timeline on it, to keep the cash cow going Blizzard has 3 directions to take:
1. Keep the classic servers open and allow players to "move on" to TBC if they wish. (Divides the player base, keeps the classic players running in a circle and eventually falling off)
2. Introduce Classic+ while keeping players in Azeroth (extends the life of current classic subs 2-3 years)
3. Force TBC migration and divide the player base
Blizzard, being a company is going to obviously focus on producing & maintaining profits. Blizzard also understanding that the linear expansion path ends with a crappy version of the game (ie: retail).
Personally, I see Blizzard going the route of:
1. 2021 - TBC Release w/ Option to play it
2. 2023 - Wotlk Releases w/ the Option to play it
3. 2021 - Classic Servers remain option and stay on Classic Version
4. 2021 - Classic+ Opens up with the option to play it
Classic+ and TBC will be announced at the same time.