I once read a fic where Tonks conquered the Portkey vertigo by removing her inner ear canal. Of course, she was deaf for a bit, but meh. I'd assume that Tonks can change her form to anything biological. This means no iron limbs or titanium skin or shit like that.

 

I've seen this in so many HONKS fics that I figure it's official fanon by now.

Pretty much.

Let's get this straight.

Level of Tonks' metamorphmagus abilities:

1. She can change skin, eye and hair color along with hair length. Facial structure can be altered, as far as muscle shape and soft tissue are concerned but reshaping bone cannot be done. Same with the rest of the body too, basically. Also, she can reshape her vocal cords to imitate any voice.
2. As above but she can change her bone structure as well, particularly in her face, though extreme changes in height and weight aren't possible.
3. As above but she CAN change her height and weight quite a bit. After all, if scrawny Harry can polyjuice into one of Draco's hulking goons using magic, Tonks can do it with her magic. She can change genders too, though the male equipment doesn't fire real ammo. She can even shrink down to imitate a child or grow to Madame Maxime proportions. However, changing her body's skeletal structure to that of an animal's along with necessary alterations of tendon and muscle placement are impossible.
4. As above and she can even control her hormones to the point where she can become fully male in every sense (Incidentally, this would likely destroy her eggs, forcing her body to replace them every time she changes back to female. Something normally impossible.), regress to a prepubescent state or advance to post-menopausal and back again. It's entirely possible she can regrow damaged limbs, move vital organs so they aren't where you'd normally expect them to be or grow extra limbs and impersonate Hindu gods... With that kind of control over her hormones, she could very well extend her lifespan at will.
5. As above, plus she can alter her form to the point where she can imitate all sorts of animals, however, she cannot change from a warm-blooded mammal to other things like cold blooded reptiles or insects. Also, while she can take the shape of, for example, a unicorn or thestral, she wouldn't display their abilities like life-giving blood or being invisible to those who haven't seen death.
6. As above but she CAN become cold-blooded, et cetera and DOES gain the animals' powers and she can change her flesh to other forms of matter, like living titanium with diamond teeth or a cloud of poisonous gas. Also, she is the only person other than Harry who can defeat Voldemort, she gave birth to the Christ child AND the sun shine's out her perfect, spankable, soft-yet-muscular arse. Hallelujah, Amen. Holy fucking shit...

I'd say she comes in somewhere between 2 and 3 in canon.

Oh, and somewhere in that list she probably becomes heavily resistant to scarring because of the way she can control her skin's movement and shape. Towards the bottom of the list, scarring wouldn't be an issue because she could basically separate any scar tissue she formed from the rest of her and regrow the missing flesh at will.

Also, her skin is probably more resistant to losing its elasticity with age, meaning that even if she became old and frail with failing health, she's still look young if she chose to.

As for her natural form: In canon, I'm fairly certain her natural form is human in appearance and it's probably the look she uses most of the time.

In fan fic, whatever works for the story, I guess.
Yes, the whole 'I think your natural form is the prettiest' thing is cliche by now. Too many fics just have him slobber over her 'real' form because he's such a sensitive, wonderful man... a little of that goes a looong way (towards insulin shock, and this is coming from a hopeless romantic). Some stories even have her real form be even more perfect that her normal one, which defeats the purpose to some extent, though I get it, really; she's avoiding the Delacour curse of uber beauty.

I mentioned here once before a fic where she didn't like her natural form because it was a dead ringer for Princess Diana (who was anachronistically already dead in that fic).

I've fiddled with a couple of scenarios regarding Tonks' real form. In one, much like her mother, her natural form very closely resembled Bellatrix, so she hated it and didn't use it.

In another, it was pretty much like canon.

In yet another, she looked pretty fucking weird, with scrawny limbs, very translucent skin and big, black pupiless eyes, sharp, fine facial features and colorless, nearly clear hair. Basically, she looked like a less extreme version of the infamous 'gray aliens', only with hair and a normal size skull. In that case, I also toyed with the idea that metamorphmagi were born looking like their parents but shortly after birth, their abilities kicked in, causing them to take on the unearthly appearance described above. Back in the day, this caused people to think their infants had been swapped for elf/faerie babies; hence the legends of 'changlings'. Because of this, many of these children were killed within weeks of birth, which is why there are nearly no metamorphmagi left in the world; they were forcibly selected out of the gene pool.

Incidentally, in that situation, Harry thought her natural form was interesting and attractive in a sort of ethereal, unearthly way, but it was definitely NOT for everyday use. His explanation to Hermione about how he could possibly be attracted to Tonks like that was along the lines of;

H: "Do you like ice cream?"
Hr: "You know I do, Harry."
H: "And you like the strawberry banana and peanut butter ice cream they have at Fortescue's?"
Hr: "Again, yes, you know that."
H: "So it's a weird flavor but you like it, right? But would you eat it every day?"
Hr: "No, not every day, but I still like it. I get the point."

(Which makes Harry as much of a xenophile as the average Trekker.)

 

I think option #3 is the most likely.

We've seen in HBP Transfiguration lessons that Human transfiguration can do quite a lot, and I've always thought of Metamophmagus' powers being that of an innate wandless ability at self human transfiguration. Plus, I's sure somewhere in canon we've seen Tonks disguised as an old lady, so she can change her height and bone structure at least a little bit.

I suppose the pig's nose can be explained away in terms of the fact that it's a superficial change, and not really a pig's nose: it's a human's nose shaped like a pig's.