Just to add a bit: in my country blinking your emergency lights (when all of your turn signals are going) for 2-3 times as a thank you is very common. It's even considered rude not to use them e.g. after you were let in.
I don't know if that is something in other countries. I live in Hungary.
It's not exactly universal. In Germany, this can be interpreted as you warning of a pending emergency and trigger slowdown; they'd do thanks by signalling left-right-left-right rapidly.
In the US, truck drivers sometimes do this as a "thank you" if you flash your brights to them to let them know they can safely change lanes to the right after they've passed you (which is a nice thing to do for them since they can't really see shit behind on their right).
More complicated extension to this. I want a collection of standard of messages that I can send to other drivers while driving (like 6 buttons max). Some standard low distance wireless (probably based on or just Bluetooth)
Like:
"Hey check your car its got something wrong with it"
"Hey you're in my lane"
"Hey slow down it's dangerous here"
"Hey watch out road hazard ahead"
"Go ahead"
"I'm going now"
etc
Just flashing lights, horns, hand gestures, and yelling out my window is a freaking game of charades man!