How proud were you when you received the super rare item in a raid far off in the past? or just the fact that you cleared the content? Wouldn't that be great to relive those times, and give others who didn't have the chance the same opportunity? As we all know blizzard is giving us that experience with Classic.
With all the politics aside, lets assume that this will be a fairly well balanced release of the game with a few servers that have most the features people want. What this is is a "remaster" that can present the opportunity to receive rewards for the current live version of WoW. this can lead into the achievement system of the main account (i.E. a toggle could be placed on classic wow to choose between seeing the acheivement UI or not) and it would show on your account wide achievement list as feats of strength from X account. This would not only give a reward to the people already planning on playing -- but the intrigue for the players who are either bored, or weren't interested before they knew it could affect their live subscription and thus receiving achievement AND reward alike. (i.E clearing naxxramas several times to receieve a full set of Tier 3)
with that said, how do you keep a community going? how is this sustained? With the release of each expansion the additional feature of having the preserved storylines, quest areas etc gives the opportunity to expand more immersively into the living world of azeroth. new players are coming to join this community of WoW and subscribing to not only new and exciting content, but previous version of that content as well. This could have huge implications on how the world scape is played, during one expansion lifecycle, areas could change and have new more immersive story content that feels more like the original RTS story modes and having an almost direct impact to the story. With the narrative breakthroughs of legion and the addition of the "communities" implementation, we could be seeing a vibrant game community that is open to each player to do whatever they wanted within the warcraft universe.
Joe Schmoe has been playing since WotLK and always wanted to play through the original raid tiers of classic wow. He wants to play classic but doesn't feel like he would be fully committed, He loves playing the current content and wants to progress his current character. When he talks to his friend Jane Doe who had quit after vanilla ended and is playing on the classic Server, she entices Joe to come over and play. He realizes that with the progression earns rewards for his live account, Joe plays thru max level and raids in the off time from Live. after a while Joe convices Jane to do the same with Live wow, since the subscription is the same, she purchases and downloads the current expansion title and experiences the new game with the newly added areas that she had leveled in vanilla in featuring new story content and landscape changes. Crazy how the Horde took over stormwind huh. JK but the thought of something like that happening and playing through how that happened and how 2 expansions later the Dark Iron dwarven clan retook stormwind but forged it in their image and is now called Orzammar (lol) you get my picture.
and before you tell me "but population on the servers wouldn't work with every expansion being released" I call BS because there are thriving private server communities out there who seek blizz like standards and thrive just fine through their released content and who want to hang around, and you can see it with each expansion all the way up to mists. With the added benefit of the subscription most people would argue that it was worth the price to dabble in which ever expansion they choose. I for one LOVE the idea of being able play and progress through TBC and reliving my shaman totem dancing and mana management, and then popping over to live wow and be able to progress through the current story with all my beloved characters. Each expansion should receive a classic up to mists, and have a lead time of ~3 expansions. So after 10.0 releases, roll out wod, legion classic servers. obviously once vanilla is released, the numbers can show how many servers and what not are needed to maintain X size populations -- heck they can even see this if they get people from the Nostrolious showing their thriving community data.
On top of the subscription inclusion benefit, there is also the new "communities" that has great potential to keep players connected through diffrent servers, or game versions, allowing for there to be thriving communities of players made up of multiple guilds from X amount of game versions; 3 guilds on live, who have converged guilds on classic, classic tbc and classic wotlk for whoever wants to play those versions, and still wants to do something like progress through current content. while another guild is more casual but contributes players to the classic guilds. and so on and so forth.
TL;DR some players need incentive to play, some do not. some people want to experience change, some do not. Some players want to play it all, some, do not. blizzard should implement communities and achievements with rewards on classic expansion servers (Rewards for LIVE and the option to toggle these features UI elements for true classic play) and allow fluid gameplay through each iteration of the game with longer timed release of more recent content such as warlords+ all the player has to do is purchase the game, and pay the blanket subscription.