Fair enough - but most completely fan created stuff as opposed to licensed doesn’t have that wide a distribution so very few things rise to the level where it’s been accepted enough in the fan consciousness to be fanon. Fewer still get converted into canon. Most of the time it’s contradicted.
I suppose one example would be Ni’Var, which had its origins solely in fanzines. But that doesn’t quite work either since it wasn’t a widely known concept until DIS resurrected it.
The only thing I can think off off-hand that’s widely accepted is that the Klingons got cloaking technology from the Romulans in exchange for Klingon ship designs. But that’s been contradicted to a degree too.