First off: If you hate M+ and want to see it DIAF, that's ok, but please find another thread for discussing that.
So according to Blizzard, they see M+ as a huge success:
https://www.forbes.com/sites/hnewman.../#7e3212c67007
Great, cool, and so on. I like the system, I think a lot of other people do too.
I'm worried, because Blizzard's designers, when they're confident and happy, often walk directly into design traps that other games with similar systems have suffered from, and other games do have similar systems, most of which suck pretty hard (unlike M+).
Two primary examples of such "bad" (imho) systems - WAR's "Ward" system for dungeons, and GW2's Fractals.
With WAR, progression through endgame dungeons was utterly reliant on getting pieces of gear with "Wards" on them. If you didn't have these, you took huge extra damage which made the dungeons into wipefests. This obviously subjected you to a ton of RNG and grind, and made things incredibly linear and alt-unfriendly. They later changed the system so certain achievements granted Wards, and you didn't have to grind the armour (and could get the Wards from non-dungeon sources), but it was a pretty massive, tedious grind, and it was entirely arbitrary - instead of letting you go as far as you could by your skill and gear and coordination and so on, you had just got cock-blocked by Wards.
GW2's Fractals have a ton of issues, and to be fair, Anet have tried to improve the system, but it is a similar concept - you have some boss abilities which do "Agony" damage, which, as you progress through Fractals, goes from "meh" to "one-shot". To counter this, you have to wear gear with with slots that allow you to equip "Agony Resistance", and to get that gear, you have to grind pretty hard. That resistance serves no purpose whatsoever in the rest of the game and the Agony damage is entirely arbitrary and has at most a tenuous connection to lore (unlike in GW1). There are also enough Agony-based abilities that relying on dodging to avoid them eventually becomes non-viable. On top of this, your progression through Fractals is limited in quite a fiddly way by yet another system not even worth discussing. So again instead of progressing on skill and so on, it's mostly a giant arbitrary grind - once you get to higher ranks, loot is plentiful (including loot you'd have to have ground hard for before), and the level of Agony resist you have means it's actually easier than the middle levels. Whilst I don't think WoW will copy this, or at least pray it won't, they also have a system where instead of full dungeons, each "Fractal" consists of various semi-random dungeon-fragments (imagine if you did one boss in BRH, then one in Eye, then one in CoEN and so on), which makes the system somehow seem more repetitive and dull, rather than less. They also have an affix-style system, which is part of why I worry that Blizzard will look to this for inspiration.
So TLDR: I'm worried that M+ will get ruined by some sort of "clever" addition from Blizzard, which really amounts to arbitrary extra grind specifically for M+ and for no other purpose.
If you are worried about this or other potential M+ changes, I'd be interested to hear it - also if Blizzard would never do this, that'd be nice to hear!