Considering that Challenge Modes were basically M+'s prototype, the design challenges should be pretty well known and the actual layout design shouldn’t have required too much reinventing of the wheel.
There are a lot of things you can’t do when designing dungeons to be speed ran and we haven’t really seen much, if any of them since MoP.
The things that have been almost completely pruned out since CM’s implementation (and later M+) is things like massive unskippable RP events (like escorts) and perfectly linear dungeons with zero room for deviation, with even the most linear of dungeons having at least some wiggle room where you can invisibility pot or otherwise skirt around some packs.
The main growing pains between M+ and CM wasn’t so much dungeon design. It was more designing mechanics around affixes AND to scale up instead of simply scaling down. Blackrook Hold is the perfect example of this, with affixes like Sanguine not really allowing super cramped spaces, gauntlets not really being a thing (or if they are, not interacting with mechanics like bursting, sanguine, bolstering or necrotic in any meaningful way) and big nukes being avoidable or interruptable (RIP random one shots on Tyrannical Xavius).
Almost every CM designed dungeon would make a good M+ key and vice versa. Though making a CM dungeon be M+ ready would require some extra work in tuning and coding exceptions to certain affixes (like imagine the SSB gauntlet with Necrotic. Ick)
I can’t really speak for PvP, since MoP well and truly broke the camels back in that regard. Not that I think MoP PvP was especially bad, just that I don’t think WoW’s PvP can be good, largely by virtue of it being a Blizzard game, and one that’s balanced and designed towards PvE at that.
But WoD PvP is generally well regarded. Would an extra tier have changed that? Maybe, it depends what bullcrap trinkets and other gimmicky items they would have put in the theoretical third tier.