I think the problem is that any content made into a Classic expansion will have a few things that make the content easier. Most of the content gets nerfed at some point during an expansion and currently at least Classic content has been launching on the last patch with a multitude of class balance changes and nerfs to the raid content. Barring that, we also have knowledge on what classes to stack for the best outcome of pushing that content even when undergeared because we know what world first guilds used; for example, take Heroic Nefarian from Cataclysm. This is a fight a lot of guilds struggled with until people discovered that the massive DPS buff given would snapshot for DoTs and people started stacking feral druids to push the fight down much faster. This is something that is currently known and could be applied to any Classic release of Cataclysm to bring a large amount of DoT classes for this content, especially in later patches where shadow priests were quite strong if it were to launch on the last patch of the expansion.
In terms of just being properly geared and even with current knowledge, I still think Cataclysm would hold an edge over most expansions. A lot of the fights were starting to bring in more difficult mechanics and the heroic versions of T11 content also required kiting the likes of which we haven't really seen since for Magmaw and Conclave, which a lot of players would probably struggle with if they don't have experience doing. T12 in Firelands was incredibly difficult content even when slightly OVERgeared for it, with Ragnaros in particular being a nightmare of a fight, and Dragon Soul may be a very hated and disliked raid but it had very difficult bosses for the time at least and some decent mechanics like requiring a split raid for the first boss, crazy DPS requirements for some things like Spine, etc.
I think Wrath is just lacking in terms of mechanics to really hold down the current day players because with the right raid group the fights would be easy with current knowledge and the end of expansion patch power levels for some classes. If Wrath were to launch with shadow priests having their absolutely insane shadow apparitions that they had leading into Cataclysm, the entire raid scene would be absolutely covered in shadow priests tab doting and running in circles because apparitions spawned more while moving; I could spawn enough of them to disconnect some of my raid team with the sheer quantity of them at times. It really just depends on the patch, but all in all the raid content was still fairly easy compared to anything seen these days and the end of Wrath into beginning of Cata is really when raid fights started shifting from DPS checks into actual mechanical challenges.
It also really depends on what you consider the content to be cleared. If we're just talking normal, everything is a breeze until maybe WoD/Legion content. Heroic, Cata/WoD have some difficult content. If you're talking about pushing the hardest hard modes (0 Light Yogg, Algalon) then Wrath could be difficult but there was no reason to farm 0 Light Yogg outside of farming the mount so I feel like it's not really necessary to consider the content cleared.