Transmog is antithetical to classic and should never be added. Player power and appearance are tied together for a reason. They could have designed the game and had gear not have any stats or power but they did. You are powerful and look powerful, you got the rare loot and show it off. You don’t see some random player in a onesie suit and think “Woah they must be really good and doing really hard content or a lot of dps” You think they look cool, silly, quirky and maybe bought a MTX. In retail that’s fine because that’s a whole different world, but never for classic
Why have gear at all? A big part of a character’s player progression is gear from 1 to 60 and that progression of gear’s appearance. It’s how they even designed the gear, so some weapons and armor were made for lower levels, others middle and others high late levels. Getting your first pair of shoulder armor, your first helmet, these are big moments for players. Another issue is alts where now you have low level characters in full tier raiding gear from lvl 60 because of account wide transmog. Yes there’s limitations but I highly doubt people who want transmog even care about that, they just want to look cool. No, you have to earn it. If you have transmog, you should make appearance based on achievements like unlocking great looking gear by doing tough challenges.
With transmog, you can completely get rid of gear having stats. Honestly, they can delete gear right now in retail and put it all into some system like gems that drop from bosses and quests and whatever else. You swap the gems around in your necklace of a number of limited slots or some other invisible character progression system and you can math out over its stats and any other effects. The point the gear exists is to give stats. They could’ve easily said no, let’s just put stats onto some other system and gear is just purely for appearance. That would require changing a lot of design. You normally see the player power on the character. Without it, you can’t. They could add something like ilvl, so next to a character’s player level is their ilvl. Yet you still have the issue of ilvl pumping, or wearing bad gear for you but it has a higher ilvl/gs like you might find in wrath, then leading to more restrictied itemization as well more simpler itemization. Plus, you might lose specific unique effects from weapons, thus ruining what makes items more special and interesting in classic. You see a warrior with the unstoppable force and you know they have a chance to stun. Edge of insanity has its big proc damage. It leads to doing less interesting itemization like retail where trinkets are the most interesting things. Good thing they brought tier sets back. It’s another way to design a game, but its definitely not classic’s original design and intent.
Here’s an alternative to transmog. Costumes. Transmog gives players a reason to collect items for appearances. Costumes, is creating different costume sets from different items and being able to equip them. Even since vanilla people have had RP sets. Basically, free up the bag space so they can do a 3 second cast to put on a costume, but only when they are out of combat and not in instanced content, like bgs, dungeons and raids. Once you enter combat, the costume is taken off. You just need a little indicator that they’re in a costume. It still complicates things because let’s say you’re a rogue and want to pvp in the world. You see some warrior and they have some terrible looking transmog or costume on, so you decide to attack them, then you find out they’re in full naxx bis and you’re about to get wrecked. It just really changes things in the classic setting and won’t work as the way it is implemented in retail.
Transmog at the end of the day just provides a layer of customization but sacrifices other things for it. Players can always get more customization in many other ways. The actual face, hair customization. Mount customization. Player housing. Guild hall. Tinted armor customization. Most of these people want to just afk in cities with a costume/rp set or some old set they collected anyways. My last solution is just make transmog client side only or a toggle so you don’t see people’s transmogs. It might be kind of pathetic though, like taking off goggles and seeing all the filters people put on social media to make themselves look better and feel cooler. Costumes allow people to have the customization to make cool sets of appearances but without disrupting transmog’s affect on player power-appearance relation in pvp/pve and fits the actual RP that when you’re in combat you’re using the actual armor and weapons you’re wearing. I think retail can look silly when you can have shirtless main tanks looking like clowns. Now you have to inspect everyone to actually see what they have on.