On the flip side, you've got the EVE Online model. Expansions are released every 6-8 months as "super-patches". Like every MMO it's mandatory to be playing on the latest version, but everyone gets it for "free".

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.