I know they are never gonna do it, but I think raids should have rising difficulty within them. Like Vanilla Naxxramas. First few bosses aren't that hard, then it gets progressively harder and only dedicated raiders can clear the whole thing. That way everybody gets to see the content. But at the same time, content feels challenging, there is always something to look forward to and you don't force people to kill the same bosses on several difficulties. In addition to that, have casual raids like Zul'gurub, AQ20 and Zul'aman. Vanilla and TBC really had it perfect.