A headcanon is something that you can or cannot prove to be canon.

"Bellatrix is good at playing piano."

You can't prove it to be true, but you can't prove it to be false. So post all you want.

"He'd hate to admit it, but Lucius Malfoy secretly loves watching SpongeBob."

"Ronald Weasley has the same breakfast every morning: Lucky Charms and a choclate poptart."

"Neville Longbottom is secretly a very good artist."