The alter self spell (PHB, pp. 211-212) includes the following on changing your appearance:
Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also can't appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can't use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.
Per the Change Appearance option of alter self, you can make yourself appear as a member of another race. Could you stop that change midway, i.e. shifting to Tabaxi but stopping to where you have the ears, tail, and claws but aren't fully transformed, making yourself appear as a mixture between two races?
Would this be a valid use of the spell?