Yes the Diablo 3 rift system would be best.
I was kinda hoping it will be like that when they anounced m+
I love 5man dungeons in wow, but regular mythic is relevant only 1st week, and then it is this lfg insanity, where we just sit for hours clicking on grps and hoping for invite.
Let ppl run and raise their shadowmoon only if they like to, score still will be about doing them all, but ppl will atleast get to play the mode.
I wouldnt even mind automated system that would put ppl with similar score together.