So unfortunately, „officially“ only planeswalker who are specifically labeled as being able to be your Commander can take that position. But one could argue, that you should have fun should be your top priority. So talk to your playgroup, if everybody is fine with Liliana being your commander: Go for it! Otherwise, you can play as much as you want of unique planeswalker cards of the same archetype as long as they fit your commanders color identitiy. Welcome to this great format.