Oh come off it. Of course I don't work for SE, but neither do you. But I do know about game development (soz, I'm a 3D modelling and rigging girl - which is why I know the mount stuff would be intern level work

) to know that what is being asked is 100% within the realm of possibility for a AAA company. I am sick and tired of people making excuses for why we cannot have incredibly easy to implement content (because it is ALL reused assets. not a single thing would be new besides where the code is). If their netcode and infrastructure sucks so bad as to not be able to add more titles, maybe they should take a step back and fix it? If they don't have the man power to do something as easy as this, then maybe they should reallocate (do we REALLY need more perform stuff?) or get a larger budget? It is frankly starting to border on unacceptable.
And yes. I would rather they revisit old content and make QoL changes than make new "content" that dies with a day or is widely disliked. Perform? Eureka? Astrologos? Lords of Verminion? Squadron? The only new content that was worth anyone's time and garnered positive reviews was Ultimate and they've already said no more of that.
Anyways... I think it would be a good way to bridge the difficulty gap between normal mode stuff and extremes. Not every challenge mode has to be ballbreakingly difficult, and the system doesn't have to include every dungeon in the game. Just take three dungeons at first and release them with a few challenges each.