dunno the accusatives & possessives, but if I follow middle English’s gendered pronouns, we could use:
a/am/as
or
a/ar/ars

e.g.
I think a is very nice.
I met am/ar recently.
Is this as/ars dog?
A told me that the house is as/ars.
A said a would rather do it aself.

What do you think?

Reminds me of:
他, 她, 它 (ㄊㄚ, tā)

Optional title:

when you learn time is a flat circle