The weird thing is that there already is an external cable for PCIe. An 8-lane PCIe cable can beat Thunderbolt, at 16Gb/s instead of 10. (Although I'm not sure if that's bidirectional; if not, you'd have to go to a 16-lane cable.)
Yes, this is what they say about every new standard. And once you have thrown out all your working peripherals and bought all new ones, then LightningBlast is released, not compatible with Thunderbolt. This has been going on as long as I have been alive.
I know, I know: Thunderbolt is the last time people will have to throw out all their things and start over. This time really will be the last time because it is a new standard to rule over all, just like NuBus was.
And it's definitely not going to be dead on arrival like PCI-X, or not supported by drivers on the Mac for most peripherals like PCI-e. No, Thunderbolt is going to be truly universal and forever lasting world without end. If only you believe, amen.
Because serial is a trivial interface to implement in home brew electronics, just throw in a MAX232.
Even many major consumer electronics are still developed via a serial console. Sure they have fancier connectors to talk to your PC and the network but serial is so simple it's practically idiot proof. So it breaks far less often, makes new hardware much simpler to bring up, and it allows you to debug the fancier interfaces without interfering with their operations.
This is feasible and would be a good future, just like the future we live in now where USB has replaced serial ports, parallel ports and PS/2.