This is an interesting problem to tackle. I'd love to see you guys do something a little more out of the box than the usual carnival style content releases and power creep. The process of increasing gear/level cap on some interval always feels like a hampster wheel to me. Maybe I'm just burned out on the usual MMO model, but I have a few ideas nontheless!
Lateral Progression Content Releases - Add new stuff to do and ways to progress without invalidating previous progression. For example, instead of changing the level cap from 60 to 70, introduce a new mechanic for progression like a faction system or subclasses. Instead of introducing a new dungeon with sword drops that deal 25% more damage than existing swords, have a new dungeon drop augments or enchantments or a new gear slot item. New stuff to do without previous endgame accomplishments becoming obsolete.
Incentivize multiple playthroughs/characters - The bulk of the game is likely to be leveling up and gradual steady progression vs the sloped, slow end-game progression. Why not emphasize this gameplay by encouraging multiple playthroughs or characters? Meta or account wide progression, or content that certain classes clear more successfully than others. Bonuses to your camp perhaps? I'd rather try a new character/build than grind slow progression personally, and theres no remorse in 'abandoning' a character if their progress benefits my next character somehow.