An interview with Ms. 13 said that the costume design in the teaser will not be her actual outfit in the show, unfortunately.
Here's my take on this gender-swap thing. Changing the gender is first and foremost most likely a marketing thing the BBC wanted, but it's not necessarily a bad thing. The Doctor being female shouldn't change the character at all, like the Master regenerating into Missy didn't change his/her personality (until series 10 at least, although that was probably natural character progression and had nothing to do with gender). Missy was just as evil as the previous Masters and the gender change was really only used as a bombshell "BTW, I'm the Master! Plot twist!" moment.
The Doctor will probably be the same Time Lord we all know and love, and the writing will probably be top-notch with Chibnall taking over. The Doctor being female will most likely only be blatantly pointed out directly after regeneration and then things will move on as business as usual. I highly doubt the Doctor will wear a mini skirt and have Sonic Lipstick (although a formal dress for undercover work might be included). She'll probably wear some sort of stylish suit, and honestly, if 12 wasn't already wearing it, I think his blue coat with red lining would suit her well.
If they make it obvious and bang it over our heads that she's a girl for the next 3-5 years, then I'll say it's a mistake and it was just a dumb marketing ploy, but I don't think Chibnall would go there unless the BBC forced him too, and even then, I think they know how good Broadchurch is and to let him alone.
Yeah, breaks tradition, but that's happened before. The Eighth, Ninth, Tenth, and Eleventh Doctors all had romantic relationships with their companions. 1-7 didn't go anywhere near that territory. Doctor Who is always regenerating itself every few years to keep it fresh. The only traditions I think will always be there is that the Doctor is a Time Lord who stole a box and ran away and fights the Daleks every now and then. Anything else is up in the air.