I won't pretend to know the complexity of how the character creation system is implemented, so I can't speak to how much of a workload this would be. But there are a few different changes that could be made. It could be a flavor-text only option, given the preestablished precedent of WoLs being children of immigrants. Othardian Roegadyn have differences in appearance, but with the new sliders this shouldn't be too much of an issue. Patron deities are also purely flavor text, so a "none" option (which some folks have already requested) is another flavor. (As a point of personal preference, I think it should be left ambiguous whether the WoL is a descendant of immigrants or an immigrant themselves.) This also ties into my next point.

No Garleans
We are in all likelihood never going to get Garlean as a playable race. The Garlean NPCs are cobbled together using a mix of Hyur and Elezen assets and do not exist on the dev side of things. Given that they canonically cannot use magic, the rewrites for a Garlean WoL aren't something I expect to see.

Gender
I already know I'm opening a can of worms with this one, but it would be disingenuous of me to make a post about limitations and not bring this up. A very common request has been the unlocking/toggle between feminine and masculine animations, and based on what is showcased RE action skins it seems possible that this may change. Additionally, outside of the character creator, gear is still arbitrarily given completely different sculpts based on characters' gender. Tycoon bottoms, for instance, are just gilded pants for masculine rather than the FFV cosplay garters of the feminine. Type B bottoms on the other hand retain their 2B cosplay appearance.
I do not believe that squenix would go through the trouble of backtracking and finding every instance of gendered language for rewrites and rerecorded lines. Unfortunately this is something that needs to have been in mind from the beginning of development. The WoL is still treated as male by default especially in supplemental material. To retroactively go through over a decade of writing is a heavy, time-consuming task and an investment that we probably will not see. Exploring within existing limitations, unlocking voices and pronouns between character models seems to be a possible compromise.

Miscellaneous
Voices are locked to races despite hearing a wide variety of voices from NPCs. Various racial features such as black-tipped ears, tattoos, horn shapes, jewelry are locked to faceplates or hairstyles. The height window is somewhat arbitrary and can be stifling. *sigh* butt slider.