• Make transmogs slot based- not item based • Remove lvl restrictions • Remove armor type restrictions • Exception to race- in which case- make more transmog!
What do I mean by this? You have added the ability to basically remove just about anything but pants, and the world did not end. Opening new customization options will only cause players to play more and be happier. By this, I mean that if restrictions were removed, players would be encouraged to run content on more than one toon for items to create their collections and sets.
As an example, let us say the player is a troll priest. Everywhere you go, you see NPC priest trolls wearing the “ Defiler’s Mail Pauldrons ”, a tiki mask troll shoulder reward for mail wearers from, oddly, Arathi Basin. If you remove transmog restrictions (NPC’s certainly are not restricted- not to mention even players like how a Dark Iron Dwarf can wear full plate heritage armor, but be a mage), then the troll priest now makes a new toon, a hunter or shaman, and levels that toon focusing on getting those shoulders- for his cloth wearing priest! Now, whole new sets can be built by players while encouraging them to play more toons!
Ultimately, if you want the player base to feel that they have much more customization on their characters and want to promote them leveling other armor types to collect armor and weapons they can then transmog on other types, you open up a floodgate of more “_played tim_e” metric while also making folks happy.
As far as race, the heritage armors make total sense. You are a Zandalari- you’d wear those clothes- and no other race would. You are a Nightborne, BE, and so forth, you should only have certain armors available to them based on the fantasy. However, add more. For example, as much as I do not care for elves, let us use a BE player example. If one goes out to the Isle of Thunder, one finds the Sunreavers. As one can see, the elves march about with their iconic shield and glaive. So, if someone is a BE, find a way, be it an achievement or whatever, where they can unlock the look of these things. NOW, make it where anything they are holding can be mogged into that shield and weapon set.
That is why I say stop making mogs item based, but rather, make them SLOT based. This also means that as a toon levels or changes gear from raids and such, they still look the same. Even if they strip down to their knickers, they would look fully dressed. If someone wants to be naked I could see them having to go to a transmogrophier and hiding all their slots, but other than the less-than-1% of the nude player base, the vast majority like how we look and do not need or want to have to change things up.
That said, this BE example is perfect. Let us say the player is a Blood Elf Priest. They are holding a staff. Now they have unlocked the Spell breaker weapon set for all their toons, even this staff holding priest can go and transmog his weapon slot to be the shield and glaive. Whether he switches the staff for a mace and off-hand or whatever, he still holds his spell breaker shield and glaive. That is innovative and fun, and could potentially cause a lot of other playing. Let us say to wear this Spellbreaker Weapon Set one must, obviously, be a BE but how did they unlock it? Maybe they had to get a BE only achievement, like they had to go to BC content and kill both versions of Kael’thas, Become exalted with the Sunreavers in MoP, and do some other BE related material. This promotes played time metrics, causes them to have to earn something unique, AND learn about their race in the process by questing/dungeon-diving in Blood Elf specific material. There are dungeons, zones, and reputations around every race that we can play, so think up unique ways of making iconic and matching (as in, the colors match the heritage armor colors- “gold” matches the gold of the heritage armor- I’m looking at you, Zandalari!) heritage weapons or even other types of heritage armors available for us to go out into the world and discover. You could potentially even wrap up professions into it, as in, maybe different professions pertaining to different races do unique things. Like a troll leatherworker can get the ability to make a ‘ heritage Rush’kah mask strap ”. So players need to go get troll related achievements, like beating Zul’Gurub, Zul’Farrak, Zul’Aman, AND Zul’Drak, to unlock a cool new voodoo mask that matches some new heritage armor, but they also have to visit a fellow troll that is a leatherworker to have the strap made. And putting all it together- get their new mask. Again, player interaction, world engagement, played time metric, everything is wrapped up in this, and yet, people are rewarded and worked to get their look.
And, the lvl requirements should be taken off. When someone makes a new toon, anything they have collected should be available to them for transmogging from the get-go. (Again, slot based)