Death curse requires deliberation, and flat out isn't survivable.  It may have been headed into death-curse territory -- but it really was just taking a severe for LOTS of extra oomph to the spell.

Harry would have died if not for Michael's intervention, which came through a prayer to God. Also, there was deliberation in the way Harry sought more power, it just wasn't rationally motivated. You've just got to remember that the novels don't align perfectly with the RPG, so while mechanically it would likely have been a severe consequence, in the books it was probably a Death Curse that was cured through the power of God.

The difference here is the death curse deliberately tosses any limits overboard.  You could accidentally drain yourself to death, maybe, but I'd argue that without the choice to do so, your body's final reserves will go untapped, even as they dwindle to a point where they aren't enough to keep yourself alive.  Your body -- your *life* -- has a lot of energy bound into it that you can't tap into on a daily basis.  A death curse willingly shoves those limits aside, knowing it's your death.

Well, there was a point in Grave Peril where Harry cast a large enough fire spell that his heart stopped beating in his chest. It required actual divine intervention to get it going again.

It's mentioned specifically by McCoy that Mavra may have a death curse to use, so I'd consider any practitioner capable of using one with the right training.

If you go a changeling route, it could be that one of his parents struck a bargain with a more powerful Sidhe and had them erase the memory and send her on dry land to be safe. Since Sidhe rarely do what they are told, she got turned into a man with all the memories of a mortal life, but when he touches water turns back into the old mermaid self.

That might also give you lots of opportunities for plot hooks. Someone is looking for her, for example.

Generally, it's easier to finalize a character sheet the more you know about who the character is supposed to be. The powers aren't really the problem, most of the time, they just follow from a good description.

Expanding on this idea slightly, instead of being cursed to transform when in water, running water is gradually washing away the spell that keeps her transformed. By the time the game takes place the spell is nearing its end and he/she's transforming into a mermaid more and more often, often at completely random and inopportune times.

You could argue that only armour held as a defensive item would trigger on defence, since Enchanted Items usually require an action to activate. That goes counter to what was printed in Paranet Papers, however, so do what you will.

Don't get me wrong, I'm not opposed to a city in the Never-never.  But I do think it has to have an origin and purpose.

In the Dresdenverse, the Never-never is basically magical potential without shape.  The places we've seen are all shaped to purpose.  Either by mortal belief or supernatural control, they're given shape.

Now what if we said that this city of yours was a culmination of mortal belief in a lost city?  In the Dresdenverse, Atlantis was a real place.  I forget which story mentions something in context of, " not since the fall of Atlantis". 

But even if Atlantis was real, it's not tied to what humans dream of.  So maybe all of that human fascination with lost and legendary cities like  Atlantis and Mu and Camelot and El Dorado and Shangri-La found a home in the Never-never, where a city with features of each formed from the combined belief in them?

Human faith would seem to have power in the Dresdenverse, and supernatural beings would likely fight over it for themselves.  But with no way to associate themselves with the legends, they'd be left with using the city for their own purposes. 

Now, in regards to humans living in the Never-never, we know that they can't live there and consume things from there and remain human.  That's established in the books.  If a human stayed there long enough, they'd slowly lose themselves.  In an ancient WoJ on the subject, JB said that as humans eat things from the Never-never, their cells are replaced by Never-never cells from Never-never nourishment.  So if they ever went back to reality, where Never-never material turned into ectoplasm... So would they.

That's a great idea, but I've got a different take on it.

There is a city in the Nevernever that has always existed. It was there before the first vampire was created, the first sidhe born, the first angel fell to Earth. Neither Heaven nor Hell, it is something different yet similar, occupying that same thoughtspace that all universal concepts fall into. When the first mud huts were built in ancient Africa, their builders dreamt of this. When wood replaced mud and stone replaced wood, each idea was taken from this one, ideal city.

If you were to visit this city you could find yourself in a replica of a tribal village in one moment then transported to a scene straight out of a science fiction film. Every building, every town, every city ever conceived was based on something from this one, monolithic city. Yet if that is the case, why does everyone dream of this place? What connects it to mortal minds and what reason does it have for sending these visions in the first place?

My pet thought here is that the city is alive. It's the first genius loci, immensely powerful, and it's trying to have mortals build enough parts of it in their realm so that it can breach through the barrier between Earth and the Nevernever, manifesting itself in the mortal plane. Could make for an interesting end game scenario. How do you defeat a living city?

When you're designing NPCs/encounters, base their stats on the average effective skills of the PCs rather than on refresh spent. This means totalling up their skills and the bonuses they get to them from powers/stunts/weapons/armour, and then average them across the party. A decent guideline is that an encounter at around the average is going to be pretty easy with a couple of aspect invocations, average +2 is a challenge and average +4 has the chance to wipe the party if they don't have plenty of fate points to spend (so BBEG time).

There are a few means I can think of, some more realistic than others.

The easiest way would be to get a plane ticket, complete with plane that will allow a lit torch on it, and then fly around the world.

Alternately, give yourself the Wings power and Mythic Speed. There may be a slight problem with violating sovereign airspace this way, though.

Another idea, with only a slight chance of losing a head, would be to use chronomancy to slow down time and then just do it normally.

You could give yourself the ability to run tirelessly and without injury, then run it. It'd take about 41 days of non-stop running to make it around the equator at 25 mph, though that doesn't answer the ocean problem.