  Creative design is a nonlinear process where designers generate diverse ideas
in the pursuit of an open-ended goal and converge towards consensus through
iterative remixing. In contrast, AI-powered design tools often employ a linear
sequence of incremental and precise instructions to approximate design
objectives. Such operations violate customary creative design practices and
thus hinder AI agents' ability to complete creative design tasks. To explore
better human-AI co-design tools, we first summarize human designers' practices
through a formative study with 12 design experts. Taking graphic design as a
representative scenario, we formulate a nonlinear human-AI co-design framework
and develop a proof-of-concept prototype, OptiMuse. We evaluate OptiMuse and
validate the nonlinear framework through a comparative study. We notice a
subconscious change in people's attitudes towards AI agents, shifting from
perceiving them as mere executors to regarding them as opinionated colleagues.
This shift effectively fostered the exploration and reflection processes of
individual designers.
