Do you read/watch a great story/show that has characters with no confirmed gender/race/sexuality/body type? That's so cool! You know why? Because you as the viewer can come up with those yourself! Making the character more relatable to you!

That being said, not everyone is you.

Take the Steven Universe fandom for example:
The gems are alien being whose physical appearances are illusions. Most people have headcanons for the gems human appearance.
Now the most common one is headcanoning Garnet as black. I personally agree with this headcanon 100% but the thing is if you headcanon Garnet as anything else besides that, you totally can! Because she has no canon race.

But the fandom will drive you into the ground calling you racist and calling your actions "white-washing" if you come out with this opinion publicly.

Let me remind you, she has NO canon race, but if you disagree with someone else's headcanon you're wrong.

Reminder: A headcanon is your interpretation of something not yet canon in a show.

Key words: your; not yet canon

Your headcanon is yours. Don't force it on others who don't agree with you.

Another thing about headcanons is that your headcanon can't really contradict what the actual canon says.

Back to Steven Universe:
The people who headcanon Ruby as a guy, despite the show constantly using "her" and "she" when referring to Ruby.

Nice that you have that headcanon, but rubys a girl hun. The show said so. I can't change that, and neither can you.

Now that doesn't mean you can't have that headcanon! Just don't get your hopes up on it being canon.

So respect other people's headcanons and it's fine if you don't have them all figured out yet!

Like for earthbound none of the characters (besides porky) have canon body types! And I like to imagine Ness would be short and chubby, while Paula is tall lanky. But for the character Jeff, I can't figure is he'd be more on the chubby side, or if he'd be tall, or if he'd be better short.

See with headcanons you're given so many choices!

And like the game lifeline is a text based game and your there to help Taylor, a space cadet stranded on a distant moon. Now the only thing you know about Taylor is that their stranded, snarky, and really in need of your help. You have no more information about them. In your mind you see them as female? Great so do I! But it's ok if you see them as male or agender too! You know why? BECAUSE THEY HAVE NO CANON GENDERRRRRRRRR.

But remember your headcanons are yours. So don't enforce them in others!