I know there are lots of things that would be nice to add to the creator - but even if they wanted to add things, they're locked in by what framework already exists, and maybe can't expand it without rewriting a lot of code about how character information is transmitted and used. Plus there are all the NPCs that are built using the same features, so anything currently existing can't simply be changed without affecting the appearance of every NPC that uses that data.
Anything that requires introducing new options (eg. another set of choices or a slider) probably can't be added without a redesign of the system.
While I'm not really familiar with the coding, I know data is transmitted with binary code, and the amount of data that could be sent in a particular number of code-bits should increase in 'powers of two' (2,4,8,16, etc).
So, my guesses on things they could expand without mucking existing things up:
More races - we know they can add extras, because of Au Ra. There might be limited slots of how many extra races they can add in future, possibly eight (the maximum number of options that could be transmitted in a three-digit binary number).
On the other hand, they might not be able to add more clans to the existing (or future) races. The current choice of clan A or clan B would be stored as a single digit of binary data. A choice of three clans would require two digits to store that information - and so I assume it could not be added.
More faces and face elements - currently most races have four face options, up to six for male Midlanders, and face elements such as eyes and noses often come in groups of six. There might be programming space to increase each of these things to eight options without breaking anything.
More colours - if they can make space in their UI for it, additional colours should be possible - though the palette might look rather messy compared to the current neat gradient. The current grid is 8 x 24 so there should be space for up to 32 rows, and looking through character data I've noticed there are already some NPCs with "off-the-grid" colour references: eg. Curious Gorge from the WAR questline, canonically a Hellsguard, is actually a Sea Wolf with a skin colour that players can't access. (Grid reference 31,1 for a fairly dark brown - so probably not a neatly arranged set of colours like the ones we can see. Which might be why they don't want to unlock it because they can't easily straighten it out.)
More hairstyles and facepaints - I don't think they can add in the option to flip hairstyles now, but they *could* add some flipped styles as separate designs. On the other hand there must be an eventual "upper limit" to how many styles they can add, though hopefully a large one if they planned ahead for it.
Thinking it through, it might actually have been good planning to not include the option to flip hairstyles. Say they allowed "seven digits" of data for transmitting hairstyle information. That would allow for either 128 choices, or 64 choices and one digit to indicate "straight or flipped" - which would be wasted on symmetrical hairstyles. Of course, it would still be very nice if they'd give us some flipped versions of popular hairstyles as separate options. Perhaps they're conscious of the risk of giving us too many things now and running out of space later on.
Facepaints do have a flip option, and might not have been allocated as much data to begin with. I wonder how many we currently have, including the few added seasonal ones?
Voices - could go up to 16 options? (but would it be worth the extra recording time and cost? Also would mess with the 'highest to lowest pitch' arrangement they generally have going with their lists of voices.)
Meanwhile, things I don't think we'll see (because it would involve 'new' data):
Au Ra horn options - they could have done this at the start instead of giving us a choice of tail shapes. They decided not to. I wonder if it was deliberate 'forced variety' so people wouldn't all gravitate to the one design of horns?
New scars/facial hair/etc. - each 'face' gets five options to switch on and off separately, although the exact options vary. Each on/off would be stored as a single bit of data. There's probably no room to add extra.
Detailed body shape adjustments - would need a fair bit more data to store extra slider settings. If they can squeeze any more data-space into the character information, I'd rather see it spent on other things. (At a guess, if a slider can be set 1-100 then it would need seven digits of data to store the setting? That same data could carry Au Ra horn choice (4 options = 2 digits), an equivalent new choice-of-four-things for other races, four new scar/makeup/etc options and a hairstyle flip option. Would that be correct?)
Ability to choose your emotes - I would absolutely love to have it, but I don't think we're going to get it. As well as extra data to transmit for a player-character, depending on how the programming works, it might "break" the pre-set emotes for NPCs in cutscenes if the animations are de-linked from their race - in which case they would have to revisit every single scene in the game and re-program the emote choice to fix it.