UPDATE EDIT -
Thanks for your input, fellow DMs. I'll offer them this instead:
Ability Score Improvement: CHA +1
Elf Weapon Training: Proficiency with the longsword, shortsword, longbow and shortbow
Appearance: You resemble elves with additional fiendish traits. Choose some of the following features: Barbed tails, fangs, forked tongues or horns.
The player will get to decide between Winged OR Fiendish Constitution. Maybe they'll get Abyssal as bonus language.
Winged: You sprout a pair of bat-like wings from your shoulder blades. You have a flying speed of 30 feet while you aren't wearing medium or heavy armor.
Shapeshifting: You can spent an action to hide your fiendish traits. This new form does not alter your appearance in any other way. In this new form, you can pass as a normal elf but you cannot use your Winged feature. Your clothing and equipment aren't changed by this trait. You stay in the new form until you use an action to revert to your true form or until you die.
Fiendish Constitution: Starting at Level 3, you gain resistant to fire.