There are many ways to co-create proposals. They typically follow a similar pattern:
Agree on the driver (or problem / opportunity / need)
Explore the topic and understand constraints
Design a proposal (often done by a smaller group)
One way to co-create proposals is to use S3’s Proposal Forming pattern.
For inspiration for steps 2 and 3, look to classic group facilitation techniques or design thinking activities.
Besides in a face-to-face workshop, you can adapt this process for online meetings. You can even use it asynchronously (and over an extended period of time) to include many people.