Something very similar already exists and it requires you to be at least 15th level.
This ability is almost identical to the Eldritch Invocation Myriad of Many Forms:
Master of Myriad Forms
Prerequisite: 15th level
You can cast alter self at will, without expending a spell slot.
Alter self says:
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.
Being able to trade a spell slot in and out for your ability makes it functionally free, and the effect is more or less the same as alter self. The only differences between this and Myriad of Many Forms is the twice per long test limit, and that your choice of other form is limited until you can pray or whatever you do with your patron.
This should be an Eldritch Invocation around 10th level.
This ability is very similar to Mask of Many Faces and Myriad of Many Forms, occupying a power level somewhere between them. Since Warlocks already have a mechanic for giving them new abilities, you should use it. This should be an Eldritch Invocation. It is usually never balanced to just hand out abilities for free, and this should have an opportunity cost – taking it over other invocations.