Let the relic scale with average item level of that class. But cap it at certain ilv. To unlock the next tier you have to beat the new primal of whatever the new patch is.

For example at 2.0 defeating the trio and using 3 mist would have gotten you the ilv90 relic. It was the ilv cap then. When Leviathan came out, to unlock your relic to be ilv 110 you need to beat Levi EX, and have an average ilv of 110. Because of average ilv, your weapon will never be ilv 115, which is what raid weapon should be.

I think this will satisfy both the grind aspect and the challenge aspect.