new colouration, comb type or colour, feather type, eggs , splice a trait from another species of bird, anything!
my picks: macaroni penguin tufts, eye spots (like a grey peacock pheasant), tuxedo colouration, lammergeier beard, blue spangling (does that exist?), and maybe also the wing 'bar' pigeons have

and of course anything more dinosaur- related non-vestigial tail, wing claws, TEETH??