One gameplay issue I'm thinking about: What does a player do while their character is in The Stack? I've got a few ideas of my own, like giving players a simple Process crew member to control (Some Function dots and standard equipment), letting them manifest as a Shade, or shuffling GM responsibilities so they get a bigger say in plot developments.
I'm also pretty fascinated with some ideas on handling the Process. I feel like treating instances as ephemeral beings that fade in and out in response to User actions, allocating and freeing up memory as needed. Admins and Users can mark exceptions as unique ones who stick around.