Stony Tark currently has 4 challenges to offer: ironman, survivalist, nightmare and resolute. I feel like there's room for additional challenges out there for (PvE-oriented) players to challenge themselves with. Below, I suggest a few, and they probably aren't too much of a hassle to implement and would give a good amount of content for the amount of time it takes for a dev to implement them.
This challenge removes xp gained from basically every source other than completing quests (the xp reward) and exploring new areas. With the current xp rates, it should work as is to reach max level.
Optional tinkering with quest xp values could also be a thing. If paired with resolute challenge (may God have mercy on your soul), the xp penalty from greyed-out quests should probably be removed to make it possible to reach max level.
This is mainly intended for pve players who like to play by partying with their mates.
In this challenge, all normal enemies become elite enemies with regards to their hitpoints and damage. (And enemies that were elite to begin with gain more hps? Could try with +50%, +100% or +200% for non-raid non-boss content, then figure out something more balanced for those group 3/4/5 boss enemies that already have 200k+ hitpoints to begin with)
If there is a technical limitation to implementing this, you could fake it by figuring out what the multipliers are between normal and elite monsters, and slapping those modifiers as a debuff to the player character: -% to damage player deals to monsters, and +damage taken from monsters.
Could be a good idea to make the enemy hitpoints also scale with party size to some degree: +50% base hp per player, so 5-man group would mean enemy has triple hitpoints? Might have to use some other value for raid content to prevent a 6M hitpoint boss suddenly having 120M hitpoints for a team of 40.
But we already have this at home, it's called nightmare more, you say? Well, no. What happens in nightmare mode is that you try to play around their increased damage output by preventing it altogether, since the enemies are basically glass cannons, so you try to alpha strike them down before they can react, or use cc/slow to keep them from doing anything. This mode would be less of a sudden death, more of an attrition thing. Would give dot-abilities chance to run their full course, so basically something entirely different.
Tunnel vision challenge
In this challenge, the player is limited to three skill/ability trees of their choosing, but they get all the skills from those three trees at appropriate levels. Talents they still have to pick and choose from those three trees as normal. So, in theory, you could choose all three warrior trees and then play a warrior... in a classless game. This one might require some work to implement, and could be necessary to adjust for pvp content to keep these characters from mingling with normal characters, if the power levels are vastly different. Don't want to see them either steamrolling everyone, or being a liability for their team, as that will just infuriate normal players.
Extra work could be put in to tie weapon proficiencies, dual-wield, shield use and armour training to certain choices. You could do this by either making it so that certain trees outright give certain proficiencies (plate for warrior_prot, paladin_prot only, dual-wield for trees that have dual-wield stuff in them) or by giving the trees an armor value, and then adding the armor value of all three chosen trees together to determine what level of armor they can wear, with melee trees giving +1, tank trees +2, and then using some logic to determine the total number you need to have to unlock leather/mail/plate and if it takes lvl 40 to unlock a higher tier.
Dangerous Outlands challenge
This would probably work best as a global realm-wide modifier, so it's probably not going to see the light of day ever.
But what it would mean that everything in Outlands (and TBC content on Azeroth) would basically be raid content - all quests Raid quests, so they can be completed with more than 5 people. Ideally this would use a pre-patch version with max lvl 60, and outlands content would be ~normal. It just becomes increasingly harder to progress in due to level difference which really ramps up in difficulty when the enemies start being 6+ levels above yours, because of how hard it is to land attacks on them. But with pets/ranged attacks ignoring glancing blows (if they still a thing), expertise aiding melee and spell hit chance helping casters, it should be doable. Would be a really interesting experiment!
No idea what the reward should be, if any, for clearing these. Could it be possible to have a soulbound vanity bag with unique-limitation so you can only have one on your character, make it scale with how many of these you complete total or together? 6slotter for the first one, improving it to 8 slot bag from doing another, 10 slots for the third and so on?