I would like the following to see:
1. have legendaries transmoggable (trust me that at the start everyone would wanna transmog those and after a week/month you wouldn't even notice this was implemented)
2. have every pre-expansion PvP piece available for transmog through honor points from a vendor. Including belts/bracers/boots. You still need to PvP for it and when it was current, people that earned these were actually special snowflakes for that expansion. So now there is a new expansion.
3. have every pre-expansion PvE piece available for transmog through Justice points from a vendor. Including belts/bracers/boots. So if you have trouble with getting a piece from the normal raids you could probably solo, you can buy the missing ones or just safe up enough points to buy the whole set. This should be expensive, perhaps more expensive then it is now (but now you don't have a complete set)
4. transmoggable enchantments
5. Moggable tabards, some tabards have special abilities but I won't wear them because they are fugly or do not match my transmog set.
6. Colouring your gear. You could go to a painter (new proffession? or just vendor) and have the gear painted in any way possible. Now you always have a matching set regardless if you miss a piece or two. Or you did like those awesome shoulders that fit with nothing, now you can equip it and it would match.