What about elite players that have done the raids numerous times to the point where they can do them for less advanced players? Under the right design, a mission that doesn't allow for help from others can be more challenging, but you're right about connecting raids with max level. Maybe have the reward for completing a Xür raid on hard with a full fireteam be a way to attain max gear. My idea for keeping the raids difficult is to use powers or exponential functions for determining the number of enemies to make the Xür raid on hard with a full fireteam the most difficult in game and a challenge worthy of max gear.