Which is why I'm asking for suggestions on what are practical applications of multi node serial busses, but I don't want to sound like I'm not coming up with ideas so I throw I2C out there, but then I'm told it's not going to work without any further suggestion of an alternate solution or ideas, so I'm back to square one to throwing ideas on the wall with no real practical knowledge of the options available, and their pro's and cons of usage. Once I can get an idea of what is required for communication, I can come up with the pin configuration, select an MCU appropriate to I/O availability, real-estate, and cost. I can come up with some small list of options. Then when I do prototype, I can dig up some arduinos out of a box and prototype within those known constrains to see practicality of my selected device implementations, and if things look promising, buy up some MCU's from the selected list and develop again. Once price point is within reason, scale, and prototype some more.
So for now, lets just say I will select whatever MCU. The current problem. Communication. What are some options that would allow multi node serial communication for 1,000+ nodes, with minimal impact to number of communication pins.