It's pretty simple to avoid the problem of having players scattered across multiple content sets and levelcaps: stop bleeding your players dry. What are subscription fees paying for if not ongoing development of additional content?
IMO you should either charge for expansions or charge subscription fees - but not both.
WoW was, from the outset, designed to railroad players through content theme parks and give a hit of dopamine just often enough to keep them playing.
The Blizzard Warcraft Store: https://us.battle.net/shop/en/product/game/wow
Information on the "WoW Token" mechanism for turning $ into gold: http://www.tentonhammer.com/guides/guide-to-buying-and-selli...
If existing players who are willing to buy new expansions whenever they are available migrate to vanilla servers, they might as well opt out of buying the expansions, since adding new content in the vanilla servers will make them non-vanilla.
Huh.
Theme Park/Resort Casino. Basically dazzle your senses while getting you to play a rigged game to massage you with variable schedule of reward. A real, living world is spoiler-proof.
I've always thought that level caps in MMOs, particularly the WoW variety slash-and-burn where you basically make a new theme park every two years and throw the last one in the dumpster in terms of player viability, is a destructive cycle not conductive to building real living worlds.