Challenging solo-friendly content with actual worthwhile gear rewards. Think something in the vein of savage difficulty, but without requiring you to herd seven cats for however many hours a week it takes you to get the job done. It may not sound like a QoL improvement per se, but it would make the game worlds better for those of us that just can't spare the time to raid like madmen anymore. I'm certain some posters will treat this as some form of blasphemy, but eh, whatever. How people get gear shouldn't matter as long as the challenges they had to overcome to obtain it match up to the item level received.
Could be as simple as allowing people to get upgrade materials for tomestone gear. That way, it doesn't upset the balance; savage raids remain required for full BiS, while those engaging with other challenging content can simply reach the maximum possible item level via fully upgrading their tomestone gear more quickly than would be possible simply by relying on 24-mans. Well, I suppose marginally less than the maximum possible item level since their weapon would always be slightly behind.
