Before you jump to my throat, just gimme five minutes to explain it.
In the light of the pillars for #SomeChanges explained by Blizzard last week, with the uncoming Achievement System (yay!) and (at last!) changes in boosting in Wotlk Classic, I think both features will be good for the health of the expansion.
1. Hardcore Mode:
Season of Mastery showed us that there is a community willing to try the Soul of Iron Mode. This is a spin to the leveling experience that - with no boosting - will make the pre Northrend content relevant trough the expansion - making the feature allign with the pillars The World is the Main Character & Nurture and Protect Social Experiences
With the Achievement System in place, you can also give cosmetic rewards - such as a title, maybe? to show your acomplishments.
If you go this route, maybe #SomeChanges to Soul of Iron would be nice. For example, making imposible to continue playing the toon if you die, like other games do.
2. Transmog System:
This would be good for two reasons. First, will make older content relevant for collectors - you can see that already in Retail. But unlike Retail, most of these content will still need groups for completion: for example, BC Raids arent soloable in Wotlk.
The second reason is to allow the wardrobe UI feature so we can reclaim space from our banks. This feature was created exactly to solve the same problem with the original Trasmog System back in Cata. If you want, you can make this a gold sink to unlock or utilize, combating inflation in the process.
Trasmog System is also allign with The World is the Main Character & Nurture and Protect Social Experiences
Both features will make the game fresh for people without hurting the rest of the community and at the end, will extend the life span of Wotlk Classic.
Sorry for any typos
/cast Blessing of Wisdom