Yep, you've read correctly. How bad should planeswalkers at common rarity be and how good could they be? Is Tibalt, the Fiend-Blooded already the top end bc there haven't been common planeswalkers as he appeared and he's a mythic now so common planeswalkers must be way worse than him (if that's possible)?
We just need the rules, but you could think about the flavor as well: Are they still legendary? What are nonlegendary planeswalkers? Why do they all have ignited sparks then?
Let's dive into that thought experiment and let's create some planeswalkers at common level. Realistic, original, you know the drill. ;)