Many languages have suppletive number on their pronouns, and some languages have a similar situation going for their case markers.
Now, what we could do with pronouns that already mark number, thus making the marking on the case markers superfluous, is shake things up! Take some forms from the singular case markers, some from the plural ones. In the first person, maybe use plural case markers for inclusive? In the third person, maybe use plural case markers for proximate-like and singular ones for obviative-like reference.
In the 2nd person, just go wild.