If they do the same thing WoW does with it's expansions, raising the level cap every 3-4 years would work. Meaning, even tho they raise the level cap and release new content with new gear at end-game stage, they don't release all of the content. Instead, they make tiers of equipment, raids, and dungeons released every 10 months-1 year. so within the same level cap, you still change equipment.

At least I liked how they did it that way.