"Before you bother to re-invent a serial communication protocol, have you thoroughly examined every one that already exists?"
All? No. I've researched several different techniques because although UART does have standard protocol, there's no one defined packet protocol. Also no, I don't have the time to "thoroughly examine(d) every one that already exists".
Also, my question was an extremely high-level one looking for base ideas so that I can consolidate a packet protocol that I can use interchangeably with I2C, SPI, UART and the custom parallel hardware interface I'm designing into a PDIP IC without redesigning (or, as you say, re-inventing) TCP that'll work on my Linux/Unix systems, as well as on micro-controller devices with as small as 2KB of RAM, so I'm not looking for an already-created exact solution.
So, your "Didn't think so" is my "I've designed a multi-I2C-bus bit-bang technology for the Arduino/ESP that I'm currently porting to the Pi/Linux, so re-inventing helps me when need be".
Re-inventing a wheel isn't necessarily a bad thing; I've done it many times, as have all experienced developers/engineers. Reverse engineering and re-engineering something is a valuable learning exercise, and sometimes, the re-invention is better than the original :)