I think challenging solo-content to get some titles/cosmetics is fine IF it is specifically designed for each individual spec.
Horrific Visions are a grand example why generic content opened the same for every spec is a total desaster for some specs in this game. Some specs outright SUFFER in Horrific Visions. They are not just weak but completely unfun to play, too.
If Blizzard would at least be honest enough to admit that these specs no longer have a place in their game and remove them - that would be fine with me! But to leave them in and PRETEND they are worth playing is just a BS move that, to put it plainly, i find INSULTING as a player.
Mage Tower was tuned for role (with the exception of Disc); that was not fair for every spec, either. Affliction Lock for example was WAY harder than MM Hunter. And for the Healers+Tanks the differences were even bigger. BUT it was “close enough” with the option to outgear them. Some specs were simply impossible for an average player without overgearing them and required total excellence to get them done - while others could be done by everybody who was physically able to move a mouse. That’s not “good”. It’s not “fair”. But as i said: Since there was the option to overgear them and still get the full reward, it was “good enough”.
Horrific Visions as solo-content are definetly NOT “good enough”. If there wasn’t the option for group play, some specs would simply be screwed. Even more so than they already are, because it hits exactly the same specs who are already weak in solo-outdoor-content.
So, if HV were solo content the weaker specs would not only be weaker in the content you need to play to get vessels to even get into the HV, no, they would then be weaker inside the HV…and because of that they would then have to play even more outdoor content to farm more vessels - which they suck at.
So they would be screwed THREEFOLD.
Call me biased, but i think getting screwed threefold is not good design.
And IF Blizzard would put in the effort to create content for 36 specs, that would mean so much workload that we would only get like 1 or 2 dungeons per expansion.
I do NOT think that this is a route worth exploring. I would argue it is the worst possible way to go with the game.