I believe there should be something like "progressive experience", allowing players to migrate from one expansion to the next one, like wotlk > cata > mop (and not wotlk > mop, or cata > wotlk).
So it feels like the experience you get when playing retail for long time, getting through seasons.
I'd like that more than updating the whole realm to another expansion, because some players just don't want that.
This idea should also be healthier for the server itself, since people is rotating towards the last expansion (or not, some of just prefer wotlk over mop for example).