Most backpack mods have some way to upgrade the backpack. I think it would be cool to also upgrade the capability of the Ender chest. Could be a slot, recipe (al la Reinforced Shulkers , achievement, etc. The mod Nether Chest adds a key slot, so maybe adding a slot like that for an upgrade would be a good idea.

Another way, to keep it vanilla, would be to leverage the idea from #5, and after the player receives that advancement, their Ender chest gets larger again. Maybe for a total size of 3x or 4x.