Jump to the next available workspace, on the current screen. If you use 1, 2 and 5, it'll jump to workspace 3.

i3-next-workspace.sh

#!/bin/bash

map_num () {
  tr , '\n' | grep '"num":' | cut -d: -f2
  # equivalent to jshon -a -e num
  echo 11
}

next_min () {
  sort -n | cat -n | awk '{ if ($1 != $2) { print $1; exit } }'
}

i3-msg -t get_workspaces | map_num | next_min | xargs i3-msg workspace number