They are each limited to a specific element, for example, the guy who controls water can do rain, fog, create lakes, etc, but he can't control ice, with limited control over steam. They are capable of pooling their abilities (Water guy and lightning guy combine powers to create thunder storms) but that's the limit.
The girl who controls energy specifically controls 'Aether', which I've translated to mean magic mostly, with limited control over energy, as magic and energy are similar. Without her weapon, which is specifically designed by the Gods no less, she wouldn't stand a chance against the laser until years after she gained her powers. While they are given basic instructions on what they can and can't do with their powers, more powerful abilities must be developed by them alone, sorta reminscent of what would happen in an RP game.

While this sounds like it was set in stone, that was actually me musing to myself and turning musing into fan-canon. Thanks hS, that helped a lot!

While the Heroes of the story don't face many people on equal level with them when they are at full power, I have included ways that each of the 18 people can be weakened, reducing their power to half, or even a quarter, of what it should be. And since all the Chosen are humans, there are their normal weaknesses which can be taken advantage of.

The good guys lose quite frequently, now that I think of it. A large part of the story is dedicated to the efforts of Maleficent, controller of the Heartless, the bad guys, and her efforts to break up the group and control them, something she actually succeeds at.

And yes, these have also been included, things that can't be overcome. They can fix droughts, but they can't fix / manipulate death, emotions and, for the most part, free will. And to a certain extent, they can't even control their powers. On confrontation with Dementors in the Harry Potter world, their powers work automatically, regardless of the person's choice.

And the balance is something I've been trying to figure out for a while too. The basic idea was to simply do the pair off thing, one evil guy for each good guy, but it felt too cliche. Their powers themselves weren't actually given to stop evil and dark powers, but rather to prevent the Universe returning to Nothing. Each of the 18 can be used as a Key to the Door to the Light, aka, Kingdom hearts, which acts as the Reset Button for the Universe, restoring areas which have been lost to the nothingness, and bringing back the people lost to the Nothingness as well. The 18 stand to represent Creation, to go against Nothing.

If it feels like I shot down each of your ideas, I hope I didn't. This actually really helped me realise how much planning I've put into it, which seems to be a lot more thorough than I thought. And you've really helped me define the limits too. Thanks Techno!

The lesser wizards / mages in fits will actually happen. Since it is a crossover, it includes the world of Emelan, found in Tamora Pierce's 'Circle of Magic' series. Magical rules in that world specifically state that messing with nature will rebound upon the user eventually, often killing them in the process.
In the world of Emelan, there is a woman called Sandrilene fa Toren, who, as well as being one of these 18 people, has the power to manipulate magic with her own, natural power. In the end, she combines the main 10 people that the story centres around, giving them the powers and abilities of the others. The sheer level of power they now wield essentially corrupts them, and they begin acting like vengeful gods, reworking the face of the planet to suit their personality, and dividing the world into territories. The things that the 10 do before they are combined are enough to make the average mage on that world twitch, but what happens afterwards would be nothing short of a miracle, even though the 10 become corrupted and ignore the reason why they have these powers in the first place.

And that is a good possibility. I'll take a think about that.
And I usually follow that pattern too, giving people awesome powers and making them useless. I sorta came to the realisation that these are epically powerful people, so stupid sorta has to be removed from the equation.

And that would be great Makari. I hadn't even remembered about those until you mentioned them. Like I said before, feel free to go through and do them whenever you'd like.

I've actually been contemplating something like this, I just dunno what others would think. At one stage, one of the characters is sent into a future to see how things 'could' turn out. That's only an idea though, it's been bouncing through my mind for a while now.

The other wya would be through the member of the team who is omniscient. She's directly limited by the Gods to only reveal information that is desperately needed at the time, but since she knows everything, she could easily break the 4th wall.

Hmm, thanks for the ideas Leto. I'll set them free into my mind and let them bounce around. *Ponders*