Fresh off the Transparent musical finale, three-time Emmy nominee Gaby Hoffmann is in talks to star opposite Joaquin Phoenix in an untitled A24 drama from writer-director Mike Mills, Collider has exclusively learned. While official plot details remain under wraps, sources say that Phoenix will play a documentary filmmaker whose latest project involves gifted children, while the story will find him bonding with his smart-yet-sensitive nephew, whose father struggles with bipolar disorder and is in the grips of a manic episode. Hoffmann's role is being kept under wraps, but one source described the character as the …