With power creep being a thing I'm sure it's difficult to think of stat upgrades on armor with each tier. I think if base stats/effects of armor don't change much in each world but have interesting set bonuses that would be rewarding and add a layer of agency to how people play.
Set bonus examples:
- Skilling speed
- cooldown reduction
- chance for extra deathnote
- DR
etc
Or even class specific bonuses like decrease hp scaling of dimensional wormhole or increase plunderous mob chance.